Been there done that with rays,pups and other fish and the Mbu grew from 10" to 18". I did it for a couple years and grew tired of acquiring the Mbu's special diet to keep it's teeth trimmed down and decided to sell it. I never had a single problem with rays or any of my fish. You have to watch your Mbu more then anything as large more active fish stress it out. Each has it's own personality and mine had the best and did fine with rays!
Never an issue with my 16" mbu over the last couple years. I've been keeping it with my scobinas and grow out pups. This thing is the most gentle fish I have. It will eat shrimp with a vaccum type bite right off the rays and has never nipped anything before. It eats shrimp once a day and gets a clam in the evenings. Just like any other fish keep it fed and watch how it reacts. If we threw out every fish that has been a problem to keep with rays for one reason or another we wouldn't have any tankmates left for them to be with.