I think it is typically personal preference from what I understand. I know ray guys that only feed pellets and their rays are healthy and breed fairly consistently.
I also know other ray guys that don’t feed pellets at all because of the mess they make, and they are also turning out pups fairly regular too.
my thoughts are as such. If you have them eating good pellets, that’s great. If you are planning to train them on other meaty foods, you may want a cleaner fish that can grab all the excess food that the rays don’t take. Unless you want to go fishing out uneaten shrimps and such from that big ol tank on a daily.
I like to do a mix of both pellet and meaty frozen foods. I have a tinfoil barb and a flagtail as my clean up crew. All of which are subject to change at my rays discretion. Lol.