Some things I've tried:
Chinese Algae Eater (CAE) - was alright while it was smaller, but around 4" became a mean SOB, would fight with mbuna, and some cichlids ended up with sucker-type marks on their sides overnight. I got rid of the CAE.
Red-tailed shark - small ones are too timid sometimes to compete well, but larger ones can be pretty assertive, so a 4'+ tank works best.
Plecos typically worked alright so long as they got food.
Loaches - clowns worked well provided they have a space to get away to. I've kept a few other botias and they worked alright so long as they weren't too small.