For substrate depth, 2-4" is fine.
9 plants might be a bit crowded (remember, they will grow in), but it won't hurt anything if you decide to go with that.
Snails... there really aren't fish that will eat them that are suitable for a 10g tank. Your best bet, if you are worried about it, is to do an alum or h2o2 dip on all plants before adding them to the tank, or you will end up with them. You might still end up with them anyway... so long as you don't over feed, they really aren't a problem. I have them in both of my 5.5 gal tanks, and occasionally will remove 5 or 6 with some forceps if I see them on the glass... but generally I just leave them be, and they don't cause problems at all. If you overfeed the tank however, they can and will get quickly out of control.