Should they make it to the main pond area, how good are koi at population
control? Or am I facing a situation where I will have a mosquito fish pond
instead of a koi pond?
At least if they are in the veg filter, I can regulate the population by
draining the filter, and keeping only a starter brood.

Well, the good news is that they get along fine. The bad news is that
the Koi won't eat them unless they happen to find themselves next to a
tasty little Koi nugget. My big Ogon has eaten several but merely by
chance.

Population control of the mosquito fish is one reason why I haven't
introduced them into my new pond; but then I don't have any other
brilliant ideas for mosquito control. One thing about those mosquito
fish...they work!

Kevin