just to clarify a 55 gallon is 48 inches long and 12 inches wide. a 75 gallon tank is just as long but is 6 inches wider, at 18 but are about an inch lower, Of course a 75 would be better but they are not as common to find, plus the stands for them are even less common.
in terms of tankmates, I'd get something that has the same diet, so that you wont have to worry about giving the wrong food to a fish. firemouth or severums would be ideal since they are also south american cichlids. you can also put in a common pleco and let it eat the algae. Although I would not suggest silver dollars as they are herbivores and blood parrots are omnivores. Trust me its really a pain having a tank with herbivores and trying to feed omnivores in it, you dont want to go that route. A clown loach, although from a different region might also work.
I get the imperssion that you are making your choices based on whats available at the local pet store. What other kinds of fish do you see there?
