Sissy wow those are some huge snails! That is a purchase I plan to make in the spring. But, since you have them in an inside aquarium, I have 2 aquariums 30 and 55, so maybe I could get them now. I was wondering if you could PM me with the person's site name on ebay that denied you to buy. I'm wondering if he has been denied, and EVERYONE is being denied to purchase from him, instead of him not allowing you. Seems very strange to me, too. Maybe you have a name that is so similar to another person that duped him that he/she doesn't want to deal with that person ever again. I was going to try to do a purchase, just to see if I got the same denial that you did.
Also, in an aquarium, the tiny snails you are referring to, if left alone, will overpopulate your aquarium VERY quickly. They do breed like rabbits. That's why the assassin snails work well, and the same for some types of fish that will feed on the snails. Typically, in an aquarium, those tiny snails are a nuisance. Your trapdoors and my mystery snails do not multiply fast.
As far as setting up an aquarium for tropical fish, I think the easiest are the live breeders - mollies (lots of types and colors), guppies, platys (lots of types and colors). Killifish are top swimmers. Angels are middle swimmers. ALWAYS get bottom feeders like plecos (plecostomus sp?), cory cats are a load of fun to watch, always busy. I have khuli loaches and horseface loaches and a dojo loach, all of which are bottom feeders, and sometimes hide under the gravel. I just bought a second fantail goldfish, Speck! I already have "Spot" in the pond, but it was too cold to try to put this new one out there, and it's all speckled like my other one. This one is mostly white/silver with red and black spots. It gets along well with the live breeders, as none of them are tail biters typically. My big tank has angels, killifish, platys, plecos, several loaches, cory cats, purple botia. 30 gal has the guppies, mollies fantail, plecos (albino and clown), cory cats and small horseface loach. I love having variety. Of course, the fish have come a few at a time. When I bought the 55 gal, I actually brought all the water home with me, too! It's what came with 5 goldfish, which started the idea for the koi/goldfish pond!
I've had aquariums since I was a child, but must admit that I never really understood the water parameters until this past year.