I can answer part of your post, and other members with direct experience of some of the Central American cichlids can supply more on the "predatory cichlid" aspect.
Angelfish. This is a shoaling fish, so a group must contain no fewer than five, and a few more would be preferable. However, that isn't possible in a 55g 4-foot tank, so five or maybe six would be it. But this is only the beginning of the tale. Within such a group, male and female will likely be present. That means a pair may form, and at that point, either the pair or the other three/four angelfish may have to go elsewhere. Obviously another pair might from from the remaining fish...and so forth. The other thing is that angelfish must select their respective mates and bond; if not, the "pair" may soon file for divorce, and that can be disastrous in such a small tank. The only exception to a group with this species is a bonded pair; the two having selected each other. Some consider one angelfish, but I am not someone who agrees with maintaining a fish contrary to its expectations. Inhumane treatment to a fish is still what it is.
Rams. The German Ram is a developed variety of Mikrogeophagus ramirezi, and this fish too must select its mate. With sufficient aquascaping to break up the area in the 4-foot tank you could have more than a bonded pair, but in that case a larger group might be preferable. Finding tankmates for this species can be tricky, because it must have warmth. A temperature of 80F (27C) is minimum, and up around 82-86F/28-30C is better for long-term health. Not every "tropical" fish can manage with that on a permanent basis.
