In my area, we have common snappers, alligator snappers, redear, and musk turtles.
I don't think 1-3 of each species per acre would hurt a healthy pond, and in fact may be more beneficial than harmful. Right now, I have about 20 redears, and they are "chelonia non-grata". Redears have a way of taking over a pond if you don't cap a few of them. They sure can eat a lot of feed pellets, too.
I think alligator and common snappers catch mostly young fish. I think the larger fish are wise to them, and can easily avoid them.
