I have not kept that species, but am very familiar with cardinal fish in general, specifically the banggai. Though they appear to school, the banggai males hold 3D territories in the water column and are intolerant of other males entering their zone. Only the females are allowed in. So if I added 6 banggai cardinals, into a 75 gallon tank, the males will try to stake out their territories, the big male will chase everyone else into the corner (outside his zone). Then the first female will be allowed in. I'd capture that pair, then allow the next male to claim the territory.
As for schooling, if you add 5 cardinal fish, you'll end up with 1 male and x females. the surplus males will be killed off and you'd end up with a harem.
SO, if you want a "school" of 5 cardinal fish, get like 8 or 10, ID the males and females as they pair up, capture them and hold them separately (sell them off), until you have your last male left and the harem. It's much easier to capture and keep the first pair and sell the surplus, assuming you have 50/50 split of M/F. good luck.
