I don't see how it's really any different from any other MMO... you don't need 99 people in a room, nor would it be productive.
Well, the point of an MMOFPS would be to recreate large scale PVP battles. If you're just doing "raids", might as well play CoD4 singleplayer... and if you're just doing small scale battles like they're planning with Resistance 2, you might as well just play any other FPS with a squad system, like BF:BC.
Right, but you still don't need 99 people in one room. Do you think everyone in an RvR zone in WAR is in the same Vent channel? As I said it wouldn't be productive... each party or squad is going to talk amongst themselves. This doesn't make it have small scale battles, it just means everyone is in their own squad as part of a large scale battle. Which is how it should be. As someone already mentioned, see Planetside. Or any other MMO, regardless of genre, that has large scale PvP.
That said, why are we talking about a Halo MMO on a console? The concepts that were leaked out are pretty clearly showing an interface built for a PC.