Dave (or anyone), I agree the turtles can't catch healthy fish. But don't you think they would go after fish eggs, like in bream beds, and frog eggs? At least when they run out of everything else to eat?
I can spot a dozen turtles at a time sunning everywhere on the lake, and I can't really see a benefit of having so many snappers.
BTW, when I read webpages of fish hatcheries, many say they only provide to new ponds and those where all of the fish have been killed off. Is that true? (I don't expect to get them for free.)
I think the "hatcheries" you checked were probably state owned and most states that stock, make your pond "Waters of the State". Private fish farms will sell whatever you order. Many fish farm operators don't know, (or forgot) how to reach goals in a sportfish pond, since their ponds are vastly different in growing fish and goals.
As for reaching a "balance", nature will always balance out, but that balance may not be anything at all of what you envision a balance to be. You can put a wide variety of species in the pond that can co-exist well, but if wanting to fish for table fare, you may need t be more specific on what main species and sizes you'd like most.