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.