[@FallenTrinity]
Hm.

Okay, for one, I need to know how big is it- sixty feet tall? six hundred? One hundred feet tall? I like feet better than meters for this because 10 feet = 1 story, and so it's easier for me to imagine how tall they are.

But here's a definite no-no:
255mm railgun.

Here's the gist of it:
The power requirements for a 255mm railgun would be enough to keep a satellite in orbit with a 100mm railgun and strike whatever the hell you wanted, whenever you wanted, two times faster and two hundred times more reactive. 

A 120mm tank cannon firing an APDSFS round- an Armor Piercing Discarding Sabot Fin Stabilized type, usually with a fair bit of depleted uranium for maximum round density- can pierce well over 500mm of steel. In other alloys, that's still at least 200mm of penetration, and that means that in order to effectively defend oneself from that 120mm, you need to either be invisible, too hard to hit, or too strong to penetrate. That means either being really heavily armored with some really advanced stuff, being protected with some really advanced APS, or being high mobile and with advanced stealth of some sort- whether it be jamming, hiding one's heat sig, etc.

And a mech is a complex piece of equipment that will be used over a tank because of its agility and adaptability- it's a helluva lot easier to put a giant metal man into a swamp and out again than a tank. That being said, weapons are going to be advanced, and there's likely very little you can do if rounds actually HIT you. That's why soldiers wear so little armor, especially in places such as the arms, legs, or neck. It'll restrict mobility, and really, if you get hit, you're going to be pretty much down, at least for a minute, or down for good. A tank could easily take a mech on in an open battle- on a desert, field, farm, whatever, tanks are going to be prized because they'll present a smaller target, can still retain effective combat capability, and in all accounts are probably harder to kill.

Mechs would be prized because they'd have the firepower and maaaaybe the armor of a tank, with the agility of a soldier in a much larger target- they'd be able to take on anything, but they'd also be very vulnerable to hits.

Thus, something so large as to be able to conceal 155mm cannons is likely far too big. It'd be too easy to hit it from orbit, or with cruise missiles, or with kinetic artillery or a stationary railgun from very far away. The requirements to fire a 255mm railgun would be immense, and the structure of the mech would have to be absolutely PHENOMENAL to withstand the recoil. Nono, there's a reason why we put all the big guns on a battleship, and not a tank- the tank would have to be superbuilt, and it'd be too expensive for such limited movement and therefore effectiveness. The Tiger-series of tanks in WWII were very good at defeating enemies in front of them, but they often reached a battle too late, and were often outflanked and eliminated by the superior mobility of a T34.

So in terms of realism?

http://distances.io/flying/ocean-city-md/long-island-city-ny
Go to this link.

You are at Point A, in Ocean City, Maryland.
Your target is at Point B, in New York City, New York. 

With a 255mm railgun, you could probably fire off two rounds before the enemy knew there was incoming ordnance.

Those two rounds would strike Long Island within five minutes, and they would have the destructive force of a very large cruise missile- each. The sort that could level one of those very dense city blocks, and then some, I'd figure. 

Your Spectre is terribly unrealistic. Everything technically makes sense, but based on the size required to house a 255mm railgun and two 155mm [i]concealed[/i] cannon? No, it's NOT realistic. It'd be easier to build a 150mm railgun into a former nuclear silo or the sort, where it was mainly pointed directly upwards but could have a very limited firing angle, by aiming itself in its hole. It could fire off a couple rounds and hit your giant mechs within a couple minutes, provided that you're within two hundred miles. If that's too much of a delay, one could just fire cruise missiles- whatever Active Protection System you have, whatever active camouflage of various sorts you're employing, it's still likely that the cruise missile will hit. You're a very, very large target, and such massive things are very hard to hide. 

More than that, they'd be really SUPER expensive.

A realistic mech wouldn't be over three stories tall, in my opinion. Then they'll just get too big to effectively hide or counter all weapons incoming, and would be too expensive to haul around and such. Even with super advanced alien technology. If you GET this super advanced alien technology, and you can mount powerful weapons on bipedal tanks, of sorts, then that means you could build bigger or better ones using the same tech on more stable platforms, such as an MBT or a stationary turret. 

Whew.
That was... I didn't mean to write that much. I just wanted to be thorough and make sure everything was addressed, and then I just started rambling.
Sorreh