You answer:

Red-tailed Sharks also do very well in 55 gals and tend to be less aggressive if they have friends (about 3 minimum), good to go with the rainbows if they’re the same size. If they’re not too territorial you could potentially add Barbs to have an active school of fish with a Hypancistrus as another center-piece fish.

I do have a bit of knowledge on some cichlids, never owned them personally but I’ve worked with African and American species.