They are often mixed in public aquariums, but I think you would need a pretty large space with very good filtration for it to work out well at home. I would not try it with a sort shell- they tend to be pretty nippy. If I was going to try to mix a turtle with a ray at home I would get a Podocnemis erythrocephala. They are the smallest Podoc (and the prettiest in my opinion), and this genus is pretty herbivorous. If you get a male it should stay a more manageable size. They are CITES listed like all the other Podocs. Legal imports do come in sometimes, but they do tend to have a pretty high price tag, usually $1000-1500.