My two cents on the topic. I don't recommend mixing the Koi and goldfish, or should I say I've learned that although they live together well, they also breed. Koi and goldfish hybrids can be decent looking fish, some points that are good are most will be sterile so you don't have to worry as much about generations of mix breedings. bad point is, hybrids of Goldfish and Koi don't go over well with more serious keepers. This is what really brought about so many haters when it came to butterfly Koi and why many shows don't allow butterfly Koi in their shows. This past year when Louisiana held it's first Koi and goldfish show we had TWO entries that the owners didn't know they had hybrids. One was entered as a Koi (The President of the club entered), turned out to be a hybrid the other was entered as a comet goldfish, (MINE!) Both were disqualified. Can you imagine showing in your first show and not knowing your entering a hybrid? Very embarrassing to say the least. I've owned my "Goldfish" for six years, started out a black longfin comet when purchased. Over the years it lost the black and slowly turned orange. I NEVER noticed the small barbell on it's right cheek/lip. The judge noticed it though and brought up a discussion about hybrids. This spring and summer I started removing all my goldfish from the koi ponds and putting them in ponds of their own. I still haven't gotten them all moved but it's a thought to think about.
When deciding on mixing Koi and plants. If your determined to keep koi and plants I suggest Butterfly or longfin fish for best beginner introductions. My personal experience has shown that short fin koi dig for roots, and the bigger they get the more a koi will dig, but long fins just cant seem to get the power to dig like standards can. To me the best plants for standard koi are the ones that can grow roots in the water column without being eaten. That list isn't the easiest to come up with due to some plants won't grow in one area and will in another, also some koi eat some rooted plants and not all koi will eat roots, so it's another you never know deal. My rambling is done, so back to the regular program.