You could go with some OB's, ahli, dragon blood, acei's, sunshine, benga yellow, jacobferibergi, stuartgrangi, eureka red, chipoke, lawanda, red shoulder, bi-colors.
That's just a few off the top of my head. Not all of them are peacocks. Some are haps and other things. Just do research on them and their aggression levels
Some of the more aggressive ones are like bumblebees, auratus, Kenyans others
As for the amount of fish generally is an inch of fish per gallon but with cichlids people tend to overstock because it lessons aggression. In a 55 if you get all your fish at about 1-2 inches you could go with about 15-16.
But you don't want to add them all at the same time as it will throw your ph off
Do you know about cycling the tank and proper ph balances and all of that good stuff