Do not be concerned about turtles, I have literally hundreds in a smaller pond and they do not effect the fish at all. If anything they might help by cleaning up dead stuff. They are also much slower than fish and won't get that many pellets once there are a bunch of fish to eat them. Wading birds like herons don't take many fish and are only annoying when they kill a bigger fish, but in your case they really aren't hurting anything although I would scare them away for about an hour after you stock anything. The only animals that will damage the pond is cormorants, anhingas, and otters (legally I can't tell you to kill them).
