I think the suggestions have been pretty good other than otos and corys. That's just wrong. You have a number of catfish and loaches to consider. I had pictus cats in with malawi cichlids for many years. They are very comfortable at high pH and can definitely handle the aggression. I really like kubotai loaches, which are similar to yoyos and striata. Any of those would work with malawis. A bristlenose plec wouldn't be bad after the tank ages a bit.
As for africans, Peacocks are an option with yellow labs. Acei, rusty, and maingano are pretty standard options, can't go wrong with those. In a 55 I'd probably stick to no more than 4 species of african. I have labeotropheus in with my yellow labs, they have a distinct look that mixes well.