Are snails on topic?
aka are snails on the menu?
Sure. Pet snails are pets, and therefore on topic.
A question about what to do with garden snails as pests would not be on topic.
I think they should be.
There is going to be a lonnnng continuum of "pets" running the gambit from dogs and cats, to birds, reptiles, amphibians, fish… right on through to things like arachnids and snails.
At this stage, I don't think we should harass people too much over the according-to-Hoyle definition of a "pet." Definitions will vary by culture, and frankly I don't see these these less conventional interests detracting from the mainstream-companion questions you find on every other site on this subject.
Generally speaking, I think it's okay to keep it simply stated: "This site welcomes questions by individuals generally seeking to provide personal care of an animal in a domestic setting." That's in contrast to, say, industrial methods for rearing of animals for agricultural use, or zoological veterinary issues, or getting rid of pests in your garden.
I think if we take a lighter approach to moderation of these questions, you'll know when the questions start to fall well beyond the expertise we've gathered here. Until then, just enjoy.