We haven't kept mosquito fish, only goldfish and koi, but our many goldfish have never seemed that territorial. I'm sure they wouldn't fight the mosquito fish, and even if the new goldfish ate some of the fry mosquito fish seem to be so incredibly prolific that it wouldn't make much difference. I'd expect some sort of balance eventually, with the size of your pond eventually limiting the total numbers, but the two sorts of fish with their rather different diets living together quite happily.