HRP would be a good choice, they are usually on the less aggro side of the cryptoheros genus, Sajica and rainbow cichlids (archocentrus) are also species that you could look into.
Personally I think you'd be better off with one pair in the 55, because of the thin width (13"). If it were a 75 with the 18" width that would be another story, but with the 13" width I'd stick with one species.
With one species, I'd recommend Cutteri, Sajica, HRP, Myrnae (Dogofwar has all of these for sale I think), even your classic store bought cons are great to watch and are easy to acquire.
Best of luck, show us what you end up doing.