I have rays with with discus, no problem. He won't even harass the tetras I though were dead but were only hiding. The other fish will take food off him and he just doesn't care. He's about 14" and has only attacked one type of fish in the year or so I've had him, other rays. I had him in with a female retic which died due to a maintenance issue, they were fine for like 6-8 months. Then I later put in 3-4" retic he beat the crap out of her. Next a 4-5" motoro, kick seven shades of hell out of him. Both saved and living in an Iso tank. When the other rays were there my giant was constantly hiding, he never did before nor since I removed them.
It's a gamble but then again any combination is. I had a 2" discus hammer away at a 10" tri color shark. Watch them carefully and keep the ray fed. However there is no guarantee the ray won't decide to kill something and develop a taste for it. Since my 14" ray had a 3" spike they can pretty much kill anything in almost any home tank.