I have had koi and western painteds together now for over 7 years and have not experienced any problems. Normal maintanence should take care of any bacteria. I am sure with koi you have a pretty good filtration system anyway. The koi should be safe from predation. The turtles will much prefer the fish food over the actual fish, and any painted that could catch and eat a koi would be a monster!!! The winter time should be fine as long as the pond does not freeze solid. If the fish can swim, the turtles are ok.