nice thread.
ok here's some help.
firefishes will never school in our tanks. they are solitary fishes in the wild and live only in mated pairs. during feedings, they form temporary groups to feed but still, only exists in pairs. so keep only one or a pair of firefish. this applies to most dartfishes.
chromis will not school well in our tanks. neither will cardinals.
for cardinals, the best will be blue eyed cardinals (Zoramia leptacanthus) or the red spot cardinals (Apongon parvulus). The latter is difficult to sustain. needs many feedings and they get scared easily. blue eye cardinals are the best.
for best schooling results, yes like what bro marcovan has said, is "safety in numbers". a group of cardinals being kept in group by a larger more intimidating fish like a tang, angel or butterfly will do the work.
think of it like a sheep dog herding a group of sheep. with the dog around, the sheep gather in a tight group. without the dog, the sheeps disperse, and for cardinals, that's when the pecking order kicks in and the dying one by one starts kicking in.
for mccosker's flasher wrasse, keeping a group of males is possible. Most flasher wrasses are peaceful and keeping multiple males of the same species will not hurt. they do not actively chase and kill like most tangs. at least that's unlikely. instead, they will flash and chase each other and display. it's nice to see but it's not peaceful schooling and the chasing could result in jumping so you need a hood.
you could keep a harem of wrasses instead. multiple females do very well together and one male to herd them. you can watch as the male participate in nuptial display as it flashes and attract the females attention.
this works for fairies and flasher wrasses. but finding female flashers and fairies are difficult since they are uncommon.
so i hope my lengthy and boring explanation helps u in picking out a scooling fish