How big is the pond? That would be my first concern. I think the koi if they are smaller would eventually become food. Redtails are eaters. They will eat anything that they can fit in their mouths. I had some lake fish in with a redtail once, they were just as big as the redtail. I was in the room late at night, heard a bunch of splashing, I woke up the next morning to find no lake fish and a fat redtail. All and all I think a red tail would be fine for a pond given it has the space, and there is not anything smaller for it to eat.