Matt: I wouldn't place Freshwater Clams in any tank less than 30 gallons (and only one in one that size) since they can die unnoticed and foul the tank in no time.
As for the snails, in a 2gal you could keep either one single or male/female of big specimen (e.g. Apple Snail) or a few mid-sized (e.g. Nerite Snails). Keep in mind that a good filtration would prevent you from doing all those water changes. When they reproduce they might climb through
HOB filter intake and end up in the impeller, so I suggest going with a small sponge filter as back-up so when you have babies in there you can remove the HOB (place it in another tank to keep bacteria alive) and let them grow.
What about sand/soil substrate with some live low light plants?
Pepetj
Santo Domingo