I started a thread about caiman lizards and rays about a month ago. Check it out, it might answer your question.
For the majority of turtles (Im a reptile guy not a ray guy...), it would be a no in the average aquarium. *However*, matamats might be a reasonable choice. They have no beaks, and therefore no bite. The only thing that goes into their mouths are things that fit. Which unless its a matmata the size of a horse, doesnt include rays... They get about 8 inches and from my guesstimate about 40 pounds. Matas mind their own business. Only thing I would worry about are claws, but rays are fast and far from stupid. Last note, to attempt this, you would probably need a pond with a sloped bottom. Matas hate swimming, and prefer to reach up for air touching the bottom. Its a pain for them to have to stretch to breathe every time. It also might provide a little seperation from rays on the turtles part.
A *possible* alternitive might be a twist-neck turtle. They get about painted turtle size and from what Ive heard arent super hyper and aggresive like American water turtles.
DISCLAIMER- Lol I have no experience keeping rays or matamatas. This is from what Ive observed simply watching them, reading, and learning right here on MFK... I dont think anything I said there would be considered wrong, but just in case...