i have experience with both brazilian (C ascellus) and Mbu's. The first i have kept with Leopoldi's, the puffer didnt last needless to say. I also have experience with Mbus and other fish (malawi's) frankly feeding time was a frenzy and the mbu wiped out my yellow labs going for other food (he bit them in half by accident i presume because he spat them out). frankly having seen the damage that can be done and knowing Mbu's inquisitive nature i wouldn't chance it. if the ray actually registers the pain of any nibble/bite/chomp where exactly do you think the spines going to go?
Interesting point to note here is that not one person has said they have kept them together for 1year + that has to mean something.
(water wise Mbu's adapt to most water conditions as they originate from the congo and can be found in Tanganika so there shouldnt be a prob there).
In a long winded way id have to say i wouldnt chance it but if theres an issue im happy to take your leo off you!