Based solely on experience, I would add fat head minnows (for forage base and mosquito eating) and yellow perch for leeches, snails, and the things snails host.

We have perch in the pond, along with walleye, black crappie, and fathead minnows. I have yet to see a leech or a snail since putting them in. Beforehand, we had thousands of snails.

A neighbor down the road has a bass/bream pond and cannot go swimming in it for all of the leeches. He is going to add a few perch to see how it goes, but his bass may be too large for the perch to establish a population to control leeches.