TKOS gave some good advice; having a snail tank fully populated and ready for the puffers is a great idea, but I don't feel it's necessary.
IME DP's don't bother with the shells of the snails, they angle their attack when feeding so that they suck the snail out of it's shell, which makes me believe they don't have teeth that can grow to the point that they're unable to eat like their much larger cousins.
In fact, I feed my DP's bloodworms (live and frozen), brine and snails as a bi-weekly treat and they're doing great. I will be setting up a snail tank soon though just to give them a more natural diet.
HTH