I don't know much (if anything) about a Mbuna tank set up, but roughly speaking they are full of rocks, aren't they? If so, then you've inadvertently created a very good habitat already. Baryancistrus xanthellus are found the in the Rio Xingu, Brazil, and it's nearby tributaries, and live within a very rocky environment. And so have evolved to graze on the surface of the rocks, eating the 'awfuchs' that grow on them. Their intestinal tracts are very long and they have comparatively small stomachs, so they need to graze on frequently.
So that it was you should try to replicate. In addition to grazing on the rock surfaces (which won't solely be enough to sustain your fish) you should provide dried foods like algae wafers/pellets and fresh vegetables like courgette/zucchini and sweet potato. Their diet should contain a good amount of algae/vegetable matter, meatier foods can be fed but not too often.
As for maintenance in the aquarium, the temperature should be quite high (about 28c) and consequently lot's of water movement for high oxygen levels.
I don't know what a 'red marble pleco' (post some pictures?) but it could cause problems like aggression and out competing it for food.