Bumble bee snails are kinda delicate when it comes to water quality. I have three in my tank and may pick up a couple more if my hermits continue to behave. I have never witnessed this snail killing behavior in my bumble bees or even had an inkling that a bumble bee snail was behind a snail death.
I've had ceriths lay eggs, nassarius reproduce (and yes, even found baby snails on the glass) along with nerites, both my tanks are filled with huge populations of baby nerites. I have never seen reproductive behavior from my bumble bees.
Bumble bees don't like nitrates, and they're not that great a cleaner, but if you like seeing them peeking in and out of your rock from time to time, 5-10 shouldn't set you back too much and should be more than enough to get what you want out of them.
Good luck,
J.