They're usually a bit nippy and such when they're 3-4", and sometimes even cannibalistic if not fed enough. That being said, you need to keep a close eye on them, and make sure you're able to separate them if need be. Axolotls are social creatures, so most of the time they're content being around others, so you shouldn't have too much to worry about as long as they all get fed enough and have enough space to themselves.