I have several dozen gambusia 'mosquito fish' in my pond to take care of housekeeping, the eat any food that the koi don't get and also eat mosquito larvae and other water bugs. Ive never had any problem with them. They are too small to be any threat to the koi and likewise, the koi don't eat them. Ive actually seen a koi gulp one and then spit it right back out.
They over winter just fine and only get to be about an inch and a half long.
Ponds are like lovers and tic-tac's You can never have just one.