The puffers will find the snails. Oh, boy will they find the snails! I had some large mystery snails in the tank once, thinking that they would make babies and would be too big for the puffers, but my tiny puffers would attack their "foot" appendage and nip little bits out of it, and basically pestered them so much that I removed them to give them a break. I do think there are people who keep large snails in with their puffers and it works out okay, but I couldn't stand it! They are relentless and will hunt anything, anything that moves. That includes earthworms, live blackworms, adult brine shrimp, anything. You could probably feed them small fry but I never tried that.
I fed my puffers frozen bloodworms as their basic diet, with snails about every 2 days. I did not really pay attention to the number of snails I put in, because I would just harvest a bunch from my other tanks and toss them in - probably 8-9 for 4 fish. You can put one in per puffer to start, and see how quickly they eat them, and judge by that.
They are really fascinating fish, and one of my favorites, and I encourage you to give it a try, especially since they can be kept in a small tank.