I used to work for a big box store. We had to carry the fish corporate told us to, no wiggle room at all. Bleah. Sometimes I would cheat and make the computer think we had certain fish (brackish) in stock when we didn't, so I didn't have to keep getting them in only to have them die in our freshwater system (or go home to a freshwater tank, only to come back dead to be replaced and/or eventually refunded). As for the mystery snails, they need to be fed. They are not great for algae control, although they may eat some algae. But, really they need to have their own food if they are to live and survive. Any kinds of sinking pellets (a variety is best), and fruits and veggies are good for them. Also, if you salt your system with freshwater aquarium salt, that is not good for them. (they can handle small amounts of salt as long as they don't come directly in contact with it). In our store, then only animals that actually turned a profit were the fish. They were sold for many times the buying price. We actually sold most of our other animals for a loss.
As for popular fish that would do well in little tanks,
the glowing danio's suggested earlier are good--normally I don't advocate fish that have been tampered with, but these were done at a genetic level, and now breed true, they haven't been injected with dye
dwarf platties are also good (they sold well at our store and were soooo cute. They were nice to have for the people who had the littlest tanks of 3-6gallons too)
Dwarf and/or honey gouramis (if you don't already carry them)
cherry barbs are pretty and colorful. They'd like a little more room, but can handle smaller tanks. I'd usually recommend three males and two females for 10g (then the males would have someone to show off for and be much redder) (of course you likely already carry these too)
We would sometimes get clown plecos in as an opportunity buy. They had a cool pattern and didn't get huge. But, they were expensive.
Any of your dwarf cories would be good too. The pandas are great. We carried them for a little while and they were very popular. Even the regular ones that don't get too large are good.
Emily