I'm also in the process of setting up a tank for pea puffers! I presume you mean Carinotetraodon Travancoricus? Theyre absolutley adorable!
In terms of snails, anything with a "soft shell" is good, like acute bladder snails and ranshorn snails. These can repopulate at a whopping speed so if I were you id remove any snails you can find in your tank at the moment and put them in a large jar with some substrate, a live plant and a lid, or a bucket woth the same setup. That way you can breed those snails without worrying about them overrunning your main tank I don't know anything about a snail that is dangerous or poisonous to them, well maybe an
assassin snail maybe but other that that you're good.
Post some pics of your tank I'd love to see! The snails can alsp be easily identifyable - just post a pic here and I'll find out
In terms of treating your puffer fish, it depends on where they are coming from, where they were caught or bred and how long they'll be in your LFS for. Your puffers may most likley be wild caught and a lot of the time LFSs will quarantine and treat new fish that are wild caught for any parasites or illnesses. Pictures pictures pictures really save lives so if you're worried about how anything looks, drop a pic and people will help
The puffers are absolutley adorable - I will say watch for their teeth as some have a fiery attitude and will actually nip YOU. 1 male to 2 femal is an excellent ratio but its difficult to sex them until they are mature so you may not end uo woth what you want. Don't keep these guys with other slow moving fish or they'll be dinner.
For diet, I'd recommend feeding live foods like
blood worms (the best!) and artemia from the start and then slowly moving onto pellets and flakes. They are picky eaters and will require the
calcium and shells of the snails to keep their teeth ground down and in check
Hope this helps! If you've any more questions please ask