you'll have plenty of pod eaters in 55G and not enough food source for all of them. if you could get frozen/pellets eating mandarins, then i think a pair is ok. i don't know about 3 in same tank.
6 firefish will fight one another and you'll be left with one or a pair if you are lucky in the end.
you'll need to target feed sun corals.
and i haven't learned about seahorses
i thought they need seahorse only tank.