I'm not 100% sure about the firefish schooling, I personnally have never seen them do it. But that doesn't mean they won't.
If you are going to go the the chromis route, I don't believe they will mix. You would at best end up with each type forming their own school.
For cardinals I have five and they all hang out together 99% of the time. The only thing to remember is they are very peaceful and do not do well with more boisterous fish. Also they do not actively swim the tank very often, usually they hang in a group over a outcrop of rock they have "claimed".
Moby