A RES will grab any fish it can get it's mouth on, hold on tight and rip it to bite size pieces with it's sharp claws and strong arms. Any sunfish or catfish is at risk unless the fish is very big and the turtle is very small.
I was surprised to find that small creek chubs work well with RES. I put a dozen 1" in my RES tanks as feeders a month ago, and they are still all there. The chubs are very fast swimmers, and extremely wary, so the turtle cannot get anywhere near them. As a bonus the chubs eat the turtle skin that sheds, so there is less turtle ring for me to keep clean.