first I'll run down the fish as individual choices:
banggai cardinals should be kept singly or in m/f pairs. groups of same-sex pairs will usually fight amongst themselves once they reach sexual maturity. pairs can and usually will breed; they're mouth brooders, so the males will hold the eggs/hatchlings in their mouths and release them at night once they're old enough. babies arent too hard to raise, I believe newly hatched brine shrimp should suffice for hatchlings' small mouths. most people either buy them in mated pairs, get a group of juvies and let them pair off as they age and seperate out the pairs, or get a single specimen and let it mature and then introduce a second specimen of the opposite sex.
clowns too should be kept in pairs of singly, IMO pairs are much more interesting. get two smaller specimens one smaller than the other. the larger should become the female, and the smaller male. they too will usually begin breeding once they've reached sexual maturity. I'd stick to one of the smaller species- percula, ocellaris, or pink skunks (listed in order of increasing aggression). clowns will become more and more aggressive as they age, biting hands and fingers put in the tank, but these three dont bother tankmates too much even when spawning and it shouldnt be too hard to keep them with others in a 55.
flame angels are pretty feisty, they are susceptible to ich, as are all dwarf angels IMO, though not so much as tangs and some others. add last, as they dont take too well to new additions. also, they appreciate established live rock on which to graze.
mandarins arent so much fragile (in fact, they're quite resistant to disease) as they are difficult to feed. they require a substantial population of pods, and are difficult to get feeding on frozen. ORA has started breeding mandarins, tragets and green I believe, though I've only seen their targets offered for sale.
firefish are some of the best SW fish, IMO. colorful, usually front and center, and inexpensive and readily availible as well as almost always healthy.
as a group, the biggest problems I see are the mandarin and flame angel. Id discount both, the mandy because of their relative difficulty with feeding and the flame because of their aggression. I hate aggression in my tanks, I like very peaceful tanks as a general rule.
personally, I'd stock:
single banggai cardinal (or a pair, if you're willing to put in the extra effort in pairing them)
pair of percula clowns (IMO the most peaceful clown, as well as the most attractive with adult coloring)
firefish (maybe a pair, though there is a chance they wont co-exist together permanently)
in addition, I would add
a shrimp goby and pistol (for a bottom dweller, great group of fish with personality and color; the pistols have very interesting behavior as well)
a blenny of some sort (tailspot, bicolor, or starry would be my choices; a very personable group of fish with interesting patterns/color/behavior)
royal gramma (another cheap, colorful, front-and-center type fish)
trio of chromis (cheap, colorful, though they might eventually widdle themselves down to a pair)
if you cant do without the flame angel, I would do:
pair of percula clowns
pair of banggai cardinals (a pair should fend better against an aggressor than a single)
flame angel
plus
shrimp goby/pistol
blenny
royal gramma