I have had my five small-ish Blue-green Chromis for about 7 months now. They school beautifully every morning when the lights go on and at any time they get scared. They are my family's favorite fish.
If my yellow tang decides to show he rules the tank, the chromis form a tight little school. They also do it when children or people who move around a lot are close to the tank. Basically, any time they feel threatened. It is really cool.
They were put into my 75 at the same time. Rarely one or two will pick at the smallest one but he doesn't seem to be bothered by the occasional short chase. I have heard many mixed reports about Chromis demonstrating schooling in aquaria. It seems to help if they have some bigger fish to distract them from aggression with each other. Maybe one of the experts will chime in here.
John