In general, bad idea.
I have had a pair of 4" Cumberland Sliders (RES subspecies) with 7 Mosquito Fish, a 6" Comet Goldfish, and a 5" Mozambique Tilapia for about 9 months now. The turtles realized the couldn't catch any of the fish back when they were tiny little things and have never regained interest, even though they can now. If you have adequate space for the fish to escape and keep the turtles well fed, you will have minimal losses. I feed them 3 different brands of pellets (including Hikari), cut up shrimp, Massivore, chopped lettuce, bits of turkey, and some other stuff that I cannot remember (its late lol).