lol...
man how many of these questions do we get on a weekly basis?
honestly
1- you can't put too many fish in there
2- there are tons of different combinations you could possibly fit inside, but at the end of the day it boils down to your preferences, your budget and what aspect of fish you like. honestly if you are interested in aggressive fish, a 55 gallon is asking for a lot of stressed out fish and i would not recommend that. it's better to either keep one mean *** MOFO in there or a few fish that can live together.
3 - do you want only cichlids?
if you are new to fish just get the typical convict, firemouth stuff. jack dempseys are ok, but still really common. red devils, trimacs etc etc will eventually have to be moved as well.
if you want to get less common CA stuff that is still manageble to keep, i say get some guys like nanoluteus, septemfasciatus, nicaraguensis, thorichthys aureus, maybe neetroplus. go do a search on each of these on the internet and find out what they look like, their sizes, their requirements (most i have listed should be fairly easy to keep, but are not quite N00b fish).
if you want to go for common SA stuff do the GT, severum stuff. oscars will outgrow that tank in a few months.
but if you want less common items there are plenty ranging from dwarf pikes, acaras, rotikel severums, discus (not N00b fish), maybe apistos if you have knowledge on their husbandry.
the list goes on. go read up, go find pictures then you will get a better idea.