I'm selling all my Tetras and just having one small Koi. Adding in Java moss, Jungle Val, and Dwarf Sag. The negative would be the Koi's eating behavior towards the plants.
Then there is no way you can safely do this in a normal home aquarium. Koi get big... really big. Any koi that is "staying small" is a stunted an unhealthy fish. You would be better off selling/rehoming the koi and keeping the tetra. They have a much smaller bioload than even a single koi. They are related to goldfish and are an even more dirty fish.