If you can't trust players to play by the rules at a special event like this, where the whole point is the limited ammo, I am not sure you can trust them to take their hits or not be twats, so at that level of policing, I'd say just give up.
I can see the argument for real cap mags only, all too easy for those 30 rounds to become 45 by an extra 3/4 pushes of the speedloader. I can also see the argument for gas only, they have a certain amount of noise to them that makes using them harder than a pretty darn quiet aeg, everyone is then level as far as ammo capacity, mag weight, noise made when engaging enemy etc.
Personally I'd love to see a night game where you have to use one of those bifrost flash thingies from your barrel, you don't have to use tracer rounds, but the flash thing would be cool if everyone had to have them.
I am a fan of gunman's filmsim rules where you're only allowed a limited number of bbs out at a time, based on what type of weapon you have, but I think the op is striving for even more than that (and great that they are - more variety means more likelihood there will be the game types I like!).