Firefish don't school, but stories of them all killing each other off for no reason are exaggerated too.
What the genus do is form small harems, with one male and 1-3 females. The harems are generally spacious, with memebrs only swimming close together when threatened.
Generally, these fish are kept in such small confines that even the extra females will begin to bully each other to establish their own space. Two males will inevitably kill each other off, but a male and female, in a large enough tank (my *guess* would be 36" long or longer) even a male and two females (probably a longer tank) will do fine.
The main problem is that firefish are notoriously hard to sex; you have to watch their behavior in the LFS... and that method is probably less-than-perfect, too.