I don't know much about the other fish that you mentioned are, so no help there! But one thing I have learned from years of gardening (with my share of failures) is if you see something over and over again it's probably used frequently because it works. If you want to try something that you don't see other people doing (and this applies to plants as well as fish) you do so knowing that it might not work as well.
Koi and goldfish are popular in garden ponds because they are colorful, friendly and hardy. They are constantly swimming and moving and will develop trust in humans given time, making them more interesting to watch - for most people! I enjoy my fish as an element of my garden pond, but I don't necessarily want to spend a lot of time caring for fish. So the ease of keeping koi and goldfish appeals to me. You may find the challenge of keeping other types of fish to be more fun for you!
Hopefully someone else has some useful first-hand experience they can share!
