I've used Formalin to cure gill Flukes & it worked very quickly. No fish deaths & the beneficial bacteria was apparently not impacted. Dosed in the prescribed dosage Formalin probably won't kill snails. Be sure to read the instructions. Meds that contain copper will kill snails and other invertebrates.
I use land plant fertilizer sticks as root tabs once in awhile and they're heavy in formalin. Never had an issue, although I wouldn't recommend doing that lol. Formalin itself shouldn't kill snails or BB if used correctly as stated above
I suggest using mechanical means to keep your snails population under control. Chemical treatments can have unintended consequences to your fish, biofilter, etc. Bristlenose plecos are scaleless fish and are quite sensitive to many medications, even salt.
You can purchase snail traps and use a piece of cucumber or zucchini to bait the trap. The snails will be attracted to the vegetables and can be remove easily. It is not necessary to remove all the snails as they are beneficial to your aquarium. However, you do need to keep the population under control.