Please never keep mbu's with rays. Have looked after plenty of mbu puffers up to 30" and would never dream of keeping them with rays. They are so inquisitive that all it will take is your ray to get spooked and bury itself, then the mbu cruises past and sees this little thing onthe sand then ! One blind stingray.
That plus the fact mbu puffers rift lake conditions for long term maintainance.