Not necessary at all. Some snails will eat plants (like Pomacea canaliculata), but the generic "hitchhiker" snails like pond snails, small ramshorns, and Malaysian trumpets won't bother healthy plants at all, other than to possibly eat algae film off the leaves.
Not necessary in the least...but that doesn't mean you won't get some hitchhikers when you introduce new plants into your tank. No matter where the source, you will most likely get hitchhikers. They are harmless except for the fact that they reproduce rapidly and can overtake your tank if care is not taken to control their population.
Snails are easily maintained through appropriate water changes, good gravel vacs, and not overfeeding. They can only reproduce readily wiht a food source. I find them to be helpful as they eat leftover food, algae, and decaying plant matter. I also consider them a good gauge of my maintenance routine.
You mentioned snails protecting your fish from deadly gas pockets. Are you going to be using a sand substrate? MTS (malaysian trumpet snails) are great sand sifting snails that are livebearers that are often kept as they help prevent anaerobic gas pockets from forming. If you have sand, and don't want snails, you can use chopsticks to stir up the sand during water changes.