Ok thanks everyone for your replies, what kind of fish could live together without any problem?
In a 55g, lots of different fish can. None of them are going to be "big" though. An assortment of fish can be put in a 55g out of many groups of fish: dwarf angels, basslets, blennies, cardinals, chromis, clowns, damsels, darts, dottybacks, gobies, hawks, wrasses. For the most part you shouldn't keep more than one from each group(there are a couple exceptions) and obviously you can't keep every group! Check out liveaquaria.com for some ideas. Each fish has a writeup and tells you the smallest tank they can be kept in.