I've used common plecos (but they may get too big) and ancistrus (their bristles may get picked on by the Mbuna). I don't stock plecos in with my Mbuna anymore, though. The water parameters are quite different than what plecos want.
Have you considered some of the hard water synodontis?
I've had an albino bristle nose and common brown pleco in with my mbuna. They include yellow tail acei, rusties, yellow labs, white top hara in a 55. They've coexisted for over a year. Just got lots of hiding places for the plecos.
I haven't had very good luck with Ancistrus in my Malawi tanks. But any of the Pterygoplichthys types do well and they are very adaptable to the hard water (just look through any South Florida or Texas waterway).
Of course they do get large and are not good algae eaters.