I think that if you really are going to build a pond, then you should keep them if you like them. Those koi are still small, and as long as you have good filtration in the tank, you can keep them in there for a little bit. Just not for a long time. My dad has a pond and he doesn't usually plant his pond until the spring (our winters are too cold to add fish to ponds in the winter. Fish hybrinate because of the temp), so if he finds koi that he likes, he puts it into a tank thats in his greenhouse until he is ready to put them in the pond. So, your koi should be fine, unless you are planning on not doing something for them for several months.