you could definatley get a dwarf lionfish! 55's are 13 inches wide right? full grown lions are about 10-11 inches, but you need a good filter. and depending on how good filtration you have you could get one of the smallereels in there. the Echidna(spelling) genus usually stay around 2' with 30 inches as a max. if you don't do a lion (or even if you just get a dwarf) you could get a yellow tang to help keep algae down.
did you already cycleyour tank? if you are still cycling then get a domino or other larger damsel. they'll get about 6" and will be too big for a dwarf lion to est when fully grown. but if you really want other fish, then leave the lion out. and don'tmix fish untill the others are fully grown because lions have huge, HUGE mouths and will fitpretty much anything their size in it. maybe not EAT it but atleast scratch it up really good so don't guess!
if you'd want to opt for smaller fish you availible options go way up!!!!!
all kinds of gobies, dwarf angels, clowns, basslets, damsels, wrasse, hawkfish, cardinals, blennies, dartfish, and inverts can be kept in there! i personally would do a breeding pair of clowns (any kind, except maybe maroons (very aggressive)) a coral beautyor flame angel, purple tang, like 3 scissortail gobies, and a longnosed hawkfish.
pm me if you want to talk some more.