Good beginner fishes for a 55g
Clownfish - Tomato, false perc, true perc (a little tougher than others), clarkii, maroon, and red saddleback.
Hawkfish - Flame, spotted, longnose, pixy, spotted, falco --- add shrimp and crabs with caution.
Dwarf lionfish
Eels - Snowflake, chainlink, and goldentail.
Psuedochromis - Neon, sunrise, springerii, bicolor, carpet eel, diadema, orchid, and strawberry --- last introduction to tank in most cases. Quite boisterous, will bully more timid tankmates.
Assessors - Marine beta, yellow, and blue.
Hamlets - Blue, indigo, butter, and shy.
Grammas - black cap and royal
Jawfish - yellowhead, bluespot, tiger, and dusky.
Cardinalfishes - Bangaii and PJ.
There are some other fishes such as blennies, damsels, gobies, and certain wrasses that I didn't mention. All these fish are pretty hardy, but some require different needs than others. Some are much more aggressive than others. Do you plan on turning this into a reef or having any inverts? Chose wisely. Bo