I think 3-4 puffers would be great. Ideally, you don't want more than one male in a tank of that size, but they can be difficult to sex. I'm not sure the puffers really have a preference for plant types but they do appreciate a heavily planted tank. So, the plants you choose are more a factor of what sort of lighting/fertilization/CO2 etc you are willing to buy. I've had a lot of luck with dwarf puffers and tiger barbs, but your tank is pretty small for the barbs. Many people have success with Oto catfish as dwarf puffer tankmates, but as they're very fragile fish you would want to wait until the tank has matured many months and developed a decent amount of algae before adding them. I keep a clown pleco with my puffers with no problem and this would probably be a good choice, as long as you had driftwood in the tank. As for the driftwood, it would be good to have a lot of it or some rock piles as well in order to create territories within the tank. The best way to breed snails is to get yourself at least a 1g container, fill it with water, add some sort of filtration or at the very least an air stone, and throw some pond snails in there. You can feed them fish food, algae wafers, algae from your display tank, anything, really, and they'll breed like crazy. They're also the easiest snails for puffers to eat, so it's a win-win situation. If you have another tank set up, you could breed the snails in that, but beware that they'll eat plants. Also, other snails and some fish will eat their egg clusters.
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