First shoal I kept with rays was (4) at around a foot each. No issues whatsoever aside from them bailing from the pond. Some of these guys were used as teacher fish for breaking Armatus. Never had aggression issues, mixed with Dorado, and Cynopotomus as well as the Armatus.
Second shoal (6) were lil guys. Kept with baby rays till about 5-6" then sold the group. No issues aside from them jumping tank on the new owner.
Current group (9) aquired at 3-4" now 6-7" and in with a half dozen midget Marbles, a little Bala shark and an MBU. Again, no issues.
I've never had aggression issues with any Odoe, and only ever lost 2, both jumpers. All groups have gotten along without so much as torn fins or missing scales. No fighting amongst any of the groups.
I can honestly say I don't think there's a better tank mate out there for rays. I've never kept so many of 1 species of fish with rays without the rays (knock on wood) ever getting a hold of one. They don't get huge either so they aren't going to out compete the rays for food, although when big groups line up to feed it may take a while before any food hits the bottom.
When considering their overall toughness, ease of cohabbing with other fish, ease of breaking onto dead food, how well groups work together and ability to stay out from under rays, I'd say it would be very tough to find better tankmates for rays.
