Hi Steve!
I have a tank (250 Gallons) With 2 motoro rays (6", 9") a Silver arowana (12") and 6 discus (4"-6") and I've never had a better tank than this one. The fish all get along nicely, even to the point where the arowana likes to hang out and swim with the discus.
The reason everyone has a different opinion on this is because it's ALL about the timing and the individual fish. I got the discus when they were probably 2"-3". The arowana was also only 6". The reason the get along is because they grew up together.
I recently put in a new discus (he was about 2" long) and the arowana immediately attacked it. Because I took the injured discus out of the 250 gal. and put him in the hospital tank, he's alive, but missing a few fins and covered with scars.
So, basically in my opinion, all of the fish go well together, but only if they are similar sized when they are all introduced.
Since they rays are such personal fish, it really depends on your ray. I got lucky and got the 2 sweetest rays in the world. They wouldn't hurt anything except blood worms.

Some rays can be very aggressive, and will go after discus. It's really up to you to judge your ray, but most motoros I've seen would be fine with discus, except this one large male i saw at an aquarium...
Sorry say so much, and sorry if it doesn't make a lot of sense, but to summarize it up, You stand a much greater chance of success if the arowana and ray are small. Get the largest discus you can. You know your fish better than anyone else, and each of these fish are different, so you're the only person who can really answer your question.
Good luck!
