Harming the ecosystem is a "relative" question. Anytime you add something new/different, it will have consequences. You just hope that it benefits you in a way that you want. I don't see any harm in trying it to achieve your goals. The trout I add in my 1 acre pond do eat leeches. It's a good way to catch them. They are a blast to watch, jumping out of the water chasing prey.
What's the warmest your water gets? This is a big question as to will they live year-round. That may also then determine how many to stock.
The water at the surface can get to 75-80 although at the bottom it provable would never reach 65 it is probably much colder than that as the pond is deep. In the winter it fully freezes over with up to 2 feet of ice. There were trout in before that did fine so im not so concerned about temp. Could i put 5-10 in to see what would happen and if they would do fine?
