I know opro uses a special impression kit and I bet the others do to. I think once the normal stuff hardened, you would have a hell of time getting it off your teeth. I don't think there's any major differences in how the mouthguard comes out. It will probably just be shaped so that you can get it on off over the braces. Just contact whichever company you plan on ordering from and they'll let you know what to do.
When I was in high school I had braces during wrestling season. At that time, I hated wearing a mouthpiece so I just went without. If all you train in is bjj I think you can get away without a mouth piece, just be prepared to get bloody lips on a regular basis. If you train any striking you'll need one for sure.
