acutallity no aquarium is big enough for any fish. you cant determine the fish's happiness at all. you think giving them a 10000g tank would make them happy. it might at all. maybe the fish prefers the wild. the atmosphere is just not the same.
fish dude: you can keep the your red bellies in a 55 for now until they become bigger (5inch-7inch) and more territorial. if you plan on leaving the P's in the 55g perm. you might just end up with 1-3 left. in my state, P's are cost more than the cichlids, i would put them in a 125. and all your cichlids in a 55. but then again maybe in your state cichlids cost more. but anyhow put the more expensive fish in the bigger tank.