I think that you need a large group of these (at least 6) around the same size for it to work. I have two left out of four. The largest is around 8-9inches and is currently in with my rays, 30in aro, cichlas and dats. He gets along fine I think only because he is the lowest on the totem pole currently in my 300. When I had him in any smaller tank, he was a prick.
I think the key is you have to have alot of them around the same size grow up together, or one of them that is not too big and not too small to go with the large monsters. My aro did try him initially though but he alluded him. And I also put him with the big guys a while back but it was too soon as he was quickly scarred...hence I removed him. When he got a little bigger and I got tired of him harassing everyone else, I put him back with the big boys. Now, everything is cool and its really cool to watch him constantly swim in the big tank with all my other monsters. He does go at it with my johanna from time to time though. But there are never any marks on either fish.
I have the other one who is only about 7in in a ray grow out tank with one other ray. Not sure what I will do with him though.