yellow tangs get large, possibly 8" long when full grown. i don't think a 55 gallon is the best tank for one of those. the same goes for a blue tang (regal or powder blue).
bio load is not the only consideration here. different fish require different habitats. fish like clownfish don't need alot of space, but a tang is a swimmer. i guess it's kinda like keeping a horse in your bedroom- it's unfair to the horse :?
fish you might want to look at are anthias, some wrasse(the fairy wrasses are reef safe), clowns of course, blennies/gobies, dragonets, damsels, firefish, dwarf angels( may nip at corals), basslets, pseudochromis, hawkfish(if you don't have any shrimp), jawfish.
there are plenty. you are best to do some research and then post a list like metalac did.
btw- metalac...you might look into dwarf lions, fu man chu lions, some of the smaller species.