I had a misfortunate experience with discus and stingrays. It was my fault- but there just wasn't enough ground space for the Ray. And the gravel wasn't quite right.
They do come from similar waters. So they should be okay. I'd seen it at a fish store- but who knows how long that tank had been up (or lasted) I'd think a HUGE tank would be the main ticket. And how awesome would that be- Discus schooling in mid-water while the rays prowled below!