I tried it with a young mbu about 5" and some 5" retic rays. Was ok for a about a month with one nip at the end of the tail of the ray and then swapped the rays for a pair of slightly bigger rectics about 6.5". The Mbu just wouldn't leave the rays alone and after an 1.5 hr I took out the mbu and rehomed him.
Shame as it was my dream combo but would never try again. Mbu can get more aggressive as they get older but you do hear of an odd gentle mbu. I have a friend who had one that wouldn't even touch live food let alone nip anything.
Just make sure you have a back up plan or spare tank!!