I have a silver aro and rays....I've kept black aros with them as well. I'd stay away from anything else as I've seen and heard horror stories due to aggression.
I agree with Gr8KarmaSF. You can do loaches with your ray and aro, but make sure the loaches are too large to eat ahead of time. Most rays and aros will outgrow loaches and they could end up being snacks if you are not careful. I have kept loaches with my ray before and its cool to see them interact. But I found that when I kept loaches with my aro they always hid (even if they were way too large to eat at 6 inches) I guess for fear of being eaten. Good luck!
A tip...a healthy ray will NEVER refuse food. Have the store feed the ray in front of you. Don't take that BS that they JUST fed the fish, so it isn't hungry.