No problems with having them with the koi at all... Our koi have never once bothered the mosquito fish we have and the mosquito are usually in the thick of things when we feed the koi. The mosquito fish in our pond thrive.
One thing i have noted and find very interesting to watch is that they almost have a daredevil nature to them and love fast currents or moving water, some of the younger, smaller ones even love "playing" with the pond skimmer we have.
It won’t hurt at all, except that your pond already has fish. The koi will serve the same purpose as the mosquito fish — they will eat any mosquito larvae. Since you already seem to have plenty of fish for your size pond, I wouldn’t bother adding any other fish right now.