Hey What kind of Fresh Water Clam do you have? Is it the Golden Asian Clam? If so it's only be about 2 inches, if not it'll get bigger, much bigger. Be prepared.
Here's a page
(or hub? I've never used that site before. :/ They call everything hubs apparently.) that has some nice information on fresh water clams.
You might want to follow her instructions and keep it in a small apple juice container filled with sand and holes poked in the side because some guy's muscles put their "feet" (what?) in-between the silicone and the glass of their 200 gallon aquarium and shattered it. :/
So yeah. I highly recommend her method with the apple juice container.
Fresh Water Clams produce ammonia and nitrite just like any other animal so you might want to take their size into consideration and use the 1 gallon per inch rule as you would with any other fish. I'd say that this probably limits you to some snails and shrimp depending on what species you bought. I'd find out soon how big it's gonna get.
But yeah until you fins that out I'd wait on planning to stock your aquarium further.
Btw how are you setting up your tank? Are you cycling, planting, or both?