I wouldnt go with the flagtail. Ive heard/seen people do it with success but Ive also heard disaster stories! I tried it and everyhting was fine for about a month and then the flagtail started sucking on my bichir and ray so he had to go!
I would suggest dats. I recently got my first two and LOVE them! Now Im addicted! hehe Oh and I dont think youd have to worry about a dat outgrowing your tank, well at least before the motoro anyway! NEver seen it done but I think payara and Acestros (fw barracude) would be nice if you had a good CLEAN source for feeders...
I second the Payara vote, if you had some good current and good feeders I don't think they would mess with the rays. Pbass should work fine. I've seen schools of discus being kept as well which might make the discus a little nervous but if you had a nice sized school say 6-10 fish and they were good size 3+" I think that would be sweet. Frank (freshwaterstingray.nl) had that for a while I think. Discus would also be a good choice if nothing else simply because they are picky about their water and need soft low ph water like the ray would prefer. Rotkeil Severums would be sweet also. ~Trent
In the past I have kept my (scobina)ray which was about 9inches in diameter with a small shoal of large discus and I never had any problems with them. Now I keep lots of different stuff with my (motoro)ray which is only about 5 inches in diameter and I havent had any problems yet. Dont know if I will when the fish get larger tho.
Right now I keep:
Theres also 2 plecos and an arowana in with it as well and they all seem to get along well. I do make sure that they are all well fed though and I think that this helps them get along. But yeah if you put aggressive eaters in with the rays you gotta make sure that the rays get their share of the food. I have to feed my rays with a turkey baster to make sure the other fish dont steal it all before the food hits the bottom.
Good luck choosing tank mates!
I have both of my rays with a group of discus. The rays are smaller in size but they have been together in the same tank for some time and I have not had any issues. I also kept a variety of rainbowfish in with a ray for awhile and they gave the tank color and I had no issues.