Sounds like you have the right kind of set up. While sliders can cause some damage to fish I personally have NEVER experience it with my animals. I have large koi in a pond with my turtles and they never harm them or even chase them in the least. THIS DOES NOT MEAN THAT IT WILL NOT HAPPEN THOUGH. Koi are generally very fast and will not be able to get away from a turtle. I would even put some minnows in the pond and allow them to establish a good breeding population before I'd put the sliders in. This way if they have the urge to eat fish they will hit on the minnows as they will be an easier thing to eat once they grab hold of them.
My turtles are so spoiled with shrimp, scallops, tilapia fillet, water plants, greens, dried shrimp and Reptomin that they don't even bother eating the minnows either. I actually looked in the pond and today and saw hundreds and hundreds of baby minnows.
They make nice schooling fish anyway and are great contrast to the big koi. So no big deal
Steve