Ok, I'm not afraid, you are worrying about nothing, which is actually a good thing. Snails are part of the rood chain, the ecosystem. They belong and do thier part to keep things balanced.

I would not go out of my way to plant them.

In my experience I have seen the opposite in bg/res. The res tend to get bigger than the bg, especially in older ponds. Mainly because they just don't breed that well. Thier numbers or percentage are smaller and they occupy a different strata and eat unique food. With smaller numbers the individual fish tend to get bigger. bg just breed. They will eat anything, everything and just take over, especially without control. You might end up with huge ratios of bg over res, but the balance will most likely be little ones, unless you have decent predation, or cull. Cody has a good writeup on producing big bg, would apply to res as well.