I agree with
tlindsey. The part I worry about is diet. Polys are obligate carnivores and really only need meaty foods. (I guess that has a lot that can be expanded upon) Mbuna are herbivores and really shouldn't be fed high protein diets. The veggie based foods you should be feeding mbuna won't hurt polys it's not going to help them either. They need a much higher protein source. That's where the problem comes in that's not good for mbuna and can be very bad actually causing death. Aside from agression from the cichlids you need to worry about your mbunas needs.