Honestly I would not attempt to keep a clam unless your tank is at least a year old. JMO
A Squasoma Clam is also an easy clam to keep as they do not require the intense lighting that other clams require!
If you do not have any flat surface to put your clam for attachment you can use a half shell and it will attach to that for more stability.
Personally as far as clam size when purchasing one I like to have mine at least 3" or bigger as they can survive more on lighting than on target feeding. Just from my own experience as others may disagree. You can put smaller clams in a bowl with green water and once it has disappeared you can put it back into the main tank.
I found that a squasoma clam moves around more than any type clam I have ever had. No explanation for this but this is what I have noticed with mine.
Yes Gina calcium should be at the right level as clams will use it up quickly in an aquarium along with the corals, etc. Clams can grow quite fast with proper calcium levels.
Here you will find alot of information on clams and their care and requirements. Great reading!
http://reefcentral.com/forums/forumd...=41&daysprune=