I wouldn't suggest the corydoras though you don't seem to be heading that way now. I kept my bronze corydoras and axelrodi in my 5 gal while I was setting up my second ten gallon, and they were extremely skittish. I had plants and a cave, but I just don't think there was enough room for them to feel secure. My betta tended not to like them either, she would nip them so she could have their food.
Mystery snails can be a lot of fun, mine tripled in size within a few months and eats out of my hand. Though he does leave a lot of mess in the tank, if I didn't clean once a week he'd make it fairly nasty quickly.
That's sad about Wal-Mart, but at least now you'll be able to get better quality fish!