[@darkmatter]A few thoughts on power-sources:

Any boosters/jet-packs/etc would need to rely on chemical rockets--as electrical/plasma engines are more suited for deep space exploration than planetary combat maneuvers. 

The mechs themselves could rely on diesel--they'd be thirsty machines but they could manage--if we're limiting them to just being able to run/sprint with jumping and acrobatics relying solely on a secondary system (which probably won't last all that long into engagements) Nuclear power and propulsion would mean mechs could stay operational for extremely long periods of time--but the problem we never solved in the real world when it came to nuclear aircraft was how to keep the pilots safe from all that radiation. Though with nukes having been neutered much of the material from those war-heads could be repurposed.

Now if we're going near sci-fi here are a few alternatives.
[hider=Advanced Flywheel Batteries]Flywheel batteries are also known as flywheel generators. They share some characteristics with homopolar generators, and some systems integrate features of both.

Flywheels batteries use a disk composed of dense materials rapidly spinning in an enclosed, near-vacuum compartment to store and generate electricity. When electricity from an outside source is applied to the battery, this interior disk is spun faster and faster. After the electricity is cut off, the disk continues to spin, "storing" the energy potential of the electricity with its rotational motion. When one wants to draw energy out of the battery again, the spinning of the disk is used to drive an electrical motor, or alternately it may be used as the motor itself. This places a load on the spinning wheel, slowing it back down.

Flywheel batteries now being developed for vehicle applications can produce peak outputs of 150 kilowatts or more, and one being researched by University of Texas' Center for Electromechanics for use in railroad engines envisions a massive 3 megawatt flywheel battery system. NASA is also developing flywheel batteries for use on the International Space Station and other future space ventures. As the technology progresses, compact and lightweight flywheel batteries for use in personal applications may emerge.

One of the great advantages of using a flywheel battery is that it can store large amounts of potential electrical power for a very long time, exceeding modern batteries both in terms of capacity and longevity. Modern flywheel batteries can store kilowatt-hours worth of electricity, and the more advanced models available today are projected to be able to store their energy for twenty years or more. As the flywheel is employed in a near-frictionless vacuum environment, there is very little to slow it down and can keep spinning for years on end.

Compare this to the best chemical batteries, which invariably store electricity for a year or two at most, and have to be carefully disposed of at the end of their operational lifetimes (typically 3 to 5 years) because they contain a number of caustic chemicals.

The amount of energy a flywheel battery can ultimately hold depends on both the mass of the flywheel and its maximum rate of spin. The more massive the wheel, the more kinetic energy it will contain for any given rotational rate. To ensure the fastest spin possible, a flywheel is suspended using magnetic bearings within a vacuum or near-vacuum chamber.

Of course, there are inevitable complications in developing more advanced versions of this kind of technology. The more massive the flywheel is, the more likely centrifugal force will try and tear it apart the faster it spins. Research into making the flywheel out of advanced composite materials and alloys that can withstand these kind of forces is ongoing, with candidate materials including wheels made out of diamond filament fibers and carbon nanotube fibers. And the more resistant to break up from centrifugal force the wheel is, the faster it can be spun, and the more energy it can store. The current flywheel champ, being developed by NASA, is capable of 60,000+ rpms. Future versions of flywheel batteries envision rpm’s in the hundreds of thousands.

Another potential problem is that these dense, rapidly spinning wheels contain a lot of kinetic energy, and should a mishap occur or the battery be badly damaged, the flywheel could be knocked loose and tear up anything in its path. For this reason, flywheel batteries have to be heavily shielded and would often be run at below peak capacity to avoid this kind of potential problem.
Also, a pair of counter-rotating flywheels in the same battery may be necessary to avoid rotational progression problems in applications where the battery may not be well-anchored. These include mobile weapon and space flight applications. Some speculation has been put forth that flywheel batteries placed on satellites and spacecraft could also double as gyroscopes as well as energy storage devices.

Besides applications in transportation and space, advanced flywheel batteries would also be useful in providing power back-ups to installations, building, and private homes; allowing communication and power distribution systems to better handle large surges in their use; provide the high current needs for all-electric or electric-enhanced construction equipment; and provide power for high-energy-consuming weapon systems like railguns, coilguns, lasers, and plasma guns.[/hider] These could be used to power the mechs themselves, with the added danger of tearing the machine apart if they're damaged.

[hider=EXPLOSIVE POWER GENERATOR (EPG) WEAPON CARTRIDGES]

[hider=A preface on Flux Compression Generators]A technology that has been researched since the Cold War, it is used in a number of laboratory and research applications where large amounts of power needs to be generated almost instantly. Flux Compression Generators(FCGs) have also been of long-standing interest to the military, as they can also be used to create an electromagnetic pulse that renders all electronics and electrical systems in the affected area inert.

An FCG basically consists of an explosive charge inserted into or around a coil of copper or other conductive wires. The wires are charged from an auxiliary power source and the charge is detonated. The explosion causes the coil to generates a brief but intense fluxed magnetic field. This magnetic field is used to produce current in the coil, which can be fed to a device or capacitors in the split second before the wires are destroyed.

In essence, the explosive ‘compresses’ the magnetic flux of the field generated by the wires, creating an additional current in the wire. Because it happens near-instantly, electrical resistance factors don’t have time to kick in. This unfettered current multiplies the strength of the existing field immensely for a brief moment. The process destroys the generator, but is capable of producing currents as great as millions of amperes in a fraction of a second, up to over 60 times that of the starting current. For truly colossal power spikes, Flux generators can be rigged in series, where the current produced by one is used as the starting current in the next.

Flux generators have a number of design and engineering challenges. For maximum effectiveness, the detonation has to occur just as the initiating current in the coil is peaking, and getting the timing on those two systems exactly right can be tricky. The devices can also generate an intense electromagnetic pulse, so they usually have to be used with heavy shielding to protect nearby electronic devices. This is in addition to the usual precautions and protections must be taken when dealing with explosive equipment. If being used to power equipment or an experiment, both the debris and waste gasses of the explosion have to be cleared before another generator can be put in place.

Because of issues with electromagnetic pulse, developing FCGs for widespread commercial use would have a number of security and legal complications that would have to be resolved first. It is therefore unlikely that we will see them in commonplace use anytime soon.[/hider]

[hider=EPGs]These are mentioned in the Traveller tabletop RPG, and would seem to be a natural outgrowth of Flux Compression Generators. Basically, FCGs would become compact and efficient enough to be integrated into portable weaponry and other equipment, from artillery and vehicle weapons to rifles and handguns.

Functionally, they would operate in many ways similar to weapon magazines. Individual cartridges would be relatively small and cylindrical. Each different weapon system would likely have its own specially-designed cartridges, depending on its power requirements, though some manufacturers may deliberately design different weapons to take the same EPG cartridges in order to simplify manufacturing and reduce costs. The cartridges would be arrayed in magazines, and would be loaded and unloaded into weapons in a similar manner.

Some weapons, such as railguns and coilguns, use projectiles as well as large amounts of current. With these technologies, the explosive charge would serve a dual purpose, both to power the weapon and to give the projectile an initial kick in velocity before it is accelerated electromagnetically. These weapons may end up having mechanical ammunition feed systems similar to modern day firearms, in order to load the EPG/projectile bundle into the barrel and then eject the spent cartridge and waste gasses after.

Other high-energy-use weapons, such as plasma guns, particle beams, and so on, would not necessarily need such mechanical feed systems. The magazine could be designed that each cartridge could be used directly in place without the need to be fed into the main gun mechanism. This would be dependent on being able to manufacture cartridge magazines durable enough to withstand numerous internal explosion without damaging the unused cartridges. An added advantage to such a system would be that the magazine could be designed to use two or more cartridges in series, with one EPG’s output serving as the starter current for the next, allowing the user to dial-up the weapon’s potential power substantially depending on how many cartridges are used at once.

There are some downsides to this system. For one, the internally-contained explosion will add a great deal of heat to weapon systems that will likely already generate quite a bit. Advanced coolant systems will have to be made even more robust and efficient to handle this, and weapons may need a cooling-down period between shots.

Second, the internal explosion will add vibrations and kickback to the weapon, though probably not as much as modern guns. Still, the user may have to steady the weapon after each shot to compensate for the recoil. Just as in some modern guns, waste gasses from the EPG could be vented at strategic points in order to help stabilize the weapon.

Also, the internal explosion will likely not be completely muffled, meaning the ideal of a completely silent beam weapon may never be realized. This will be especially true if the very energetic waste gasses are vented, resulting in a very gun-like bang with each pull of the trigger.

Because the EPGs’ detonations would be contained entirely within a properly shielded weapon and/or magazine, its environmental EMP effects would be neutralized.[/hider][/hider]