As a keeper of quite a few clams in a 55g tank you dont have enough light for anything but a Derasa or a Squamosa.. the T.Croceas and T. Maximas are extremely light demanding clams.. I have 2x250w MH
plus 2x55w PC
actinics... I keep 5 T. Croceas and 1 T. Maxima... I agree Buy both the books... When you are done reading them cover to cover then think about buying something... You are no where near ready for clams... Your tank needs to be extremely stable and mature for any coral or Clam... IMO
you are no where near ready for anyhting else going into your tank... 3-4m maybe for a nice hardy coral.. I would say 8-12m before you think about getting a clam of any species... Clams are very difficult to keep as by the time they show any signs of distress they are already going to Die...
Again.. READ READ READ READ READ and SLOWWWWWW DOWN,