Which other cichlids do you have in the tank? Providing you have nothing too aggresive, bolivian rams will do fine in a 55g, I would suggest, not knowing the types and amounts of other fish you have, a pair, but once we know the other fish in your tank might be able to advise better.
As for food, they will eat pretty much anything, flake, pellet, live and frozen foods, mine loved bloodworms, brineshrimp, prawn, daphnia.