Personally, I think around 300fps is good for ICQB, and up to 350fps for general CQB. You'd probably want mouth protection for all ICQB games, but it your CQB game is outdoors, just keep your mouth shut and you probably don't NEED it.
Stock Marui AEGs are perfect for ICQB, since they max out at around 295fps. You don't need your BB to travel 400fps when your target is only fifteen feet away.
It's all fun and games until someone gets hurt...and then it becomes sport!