Go for 4. With 3 there isnt enough of a group and aggression/exclusion could occur, 5 and you may be pushing the limits of floor space. Like Miss Dib Dabs said, go with a pair of SA cichlids and you should have a nice tank. You could also go for some things like rainbows, another active fish, allthough not SA and an ABF for the top.
I really doubt a lenght of 7" in captivity. In the wild, yes, most, if not all reach 10" out there but in aquaria it wont get any bigger than 6" I would say, with an average size of 5" or so.
In a 55g you could also go for white linded pims if you could find them, check the fish index out.