I know I'm very new to the whole fish thing, but we have a Koi as well. He is in an outdoor pond all spring, summer and fall (goes out as soon as the ice is gone, and comes in just after it starts to show up again!), and comes in only for the winter. We have a 70 G tank, and he is probably 12" long now. I know the tank is too small for him, but it is only for part of the year, and I am in the process of looking for a 150 G tank.
In there with him, I have a pleco, a dojo loach, 3 shebunkins, 5 regular pond fish, and a Red Cap Oranda. All of them get along VERY well most of the time, but I have had trouble (last spring) with one of the pondfish, and the Koi getting a bit aggressive with my Oranda. They actually consumed a good portion of his head. In fact, I had a small Oranda too, and the whole group actually ganged up on him and ate all his fins until he died.
I am told that in most cases, Koi can get along with just about anything, however, they can have bouts of aggression, as I have clearly seen. So far so good though, as all the same fish are back together, and my Red Cap is not getting bullied.
Hope my post helps some...