More than likely the mbunas will kill the crayfish and eat'm. Repeated high doses of protiens to mbunas can cause malawi bloat - hence not recommended. Some not all species of Peacocks and Haps like a high protien diet however they can get bloodthisty - turning on their tankmates for food. Years ago I'd thro a gold fish in from time to time to see the Afro's tear it up and believe me they do - I've since given up that practice after loosing fish to bloat. Risk isn't worth the reward.