Well.. If it were me I would introduce the snail first (btw your so lucky you can get them locally.. we only have mystery snail here). There isn't much a betta can do to hurt a snail (they can hide in their shells but still move around and eat) so you can see if the betta is going to bite/harass the snail. If it seems like the betta is constantly messing with the snail I would say that that fish shouldn't have tank mates and I would remove the snail. If the betta nips it only occasionally I would say that the fish is ok with snails only (because he can't hurt them). If the betta completely ignores the snail I would say that you could add the shrimp.
I always start with snails first because you can see the betta's behavior without endangering the snail.
*Be smart.. Do your research!!*