Well, here's the thing. We have railguns and lasers available now. Granted they are mounted as shipboard weapons, or in some cases on the back of large trucks, but they exist now. The next generation of United States Navy vessels seem to have the zipper problem for railguns solved and are to be utilized as less prototypical or experimental weapons and considered part of that ship's standard compliment. Particle weapons are simply high power particle accelerators that have been miniaturized. Most of what we lack currently is efficient and portable power supplies and iin some cases efficient heat management. 

In another fifty to one hundred years, these weapons are probably going to be available and used on the battlefield at the infantryman level. For a setting like this, I don't think that having these weapons available would be far-fetched or "too sci-fi" at all. As our GM said, not every unit will have every member having one or the other. Ballistic slug-throwers are just fine, cheap, and they work. I might use a railgun, particle beam/cannon, or laser in the unit as a force multiplier, like a SAW or a crew-served .50 cal, or like a Mk-19 grenade launcher. Again, not everybody would be able to do that.

The more I think about it though, Halo is a good comparison. Another good one might well be Starship Troopers, without the Bugs.