My SAPs only eat snails when they're REALLY hungry, and at that point they'll eat any kind of snail, even Melanoides. The thing with Melanoides is that SAPs appear to bite off the "foot" rather than crunch the shell, but as Fella says, I'm pretty dubious about Melanoides being a serious danger to pufferfish. Yes, there may be one or two reports of problems, but then there are people who break their teeth eating chocolate bars, and no-one says chocolate is dangerous. So you have to put these things into perspective.
SAPs do need some tooth management. But in real terms, trimming their teeth takes about two minutes of work, the actual cutting being less than 15 seconds, and the rest being about sedating the fish before it goes 'under the knife' so to speak. Compared with their innate hardiness, adaptability to a wide range of water conditions, and overall lack of aggression, SAPs are _by far_ the easiest pufferfish to keep in the freshwater hobby.
Cheers, Neale