I'll throw in a second for pea puffers. I've got a single male in a 10 gallon high tech planted. It's him, an Otocinclus, and whatever snails manage to escape his voracious appetite. Pond snails, ramshorn, bladder snails, they all get destroyed very quickly. The only snails that have any longevity in the tank are Malaysian trumpet snails since they spend a lot of time in the substrate.
My male puffer has shared his tank with a large amano and large nerites, both were left alone. Amano died after a large water change, nerites got pulled because they poop too much. Occasionally, I'll see him chase the Oto around the tank, but he's never nipped at him and the Oto is in great health. For the most part, they leave each other alone. In a 20 gallon, you could do several pea puffers, but keeping snails stocked will be a challenge.