Have you given any thought to adding a pair of weather loaches? I've kept them with goldfish. They get along well and are an interesting contrast in shape and color. Here's a link with information about them:
I've also kept African Clawed Frogs with them until the fish got big enough that it was a problem getting food down to the frogs. Here's a link with lots of information about them:
Hi
In my opinion you are overstocked already, additionally I
would not keep Koi in any sized tank.
Please consider moving the kio and some of the goldfish to a pond