Hello:
If turtles are well fed they'll usually leave larger fish in the tank with them alone. This would mostly apply to Red-Ears, Yellow-Bellies, Sliders, and Painteds (maybe Maps). While not always true I have succeeded in mixing them together. Mud Turtles and Musks can chase or leave them alone. They are just not as quick in the water. I find that Snappers and Softshells consider anything in the tank with them as fair game.
Have a Great Day!!!