I imagine it would do ok. My puffs actually stick together. There are 4 in a 23 gallon.
I heard that once they are ready to mate, males get very territorial. I feel like everyone's experience is different, but puffers love live food like snails, scuds, daphnia and usually take frozen bloodworms. Maybe your snail might be harrassed to death.
They need very clean water and they love looking at things. I made some toys for mine. I got a ball with holes and put moss in it. They look inside for living things to eat.