debbie,
In honesty, I have never heard of a "cleaner clam".
As a general rule, clams require prestine water conditions (well kept reef system set-up), and very strong lighting (hallides). They are not easy to care for unless you have a well-establshed set-up. So if I were you, I would choose to avoid the clams, depeneding on your set-up and your experience.
As for sanils, they grow their own shells, so you do not need to provide extra shells for them. The hermit crabs do require shells. Generally when they outgrow theirs, they will just find larger ones lying aorund. Depending on how big the crabs are to begin with, then the shellks would probably be fine.
HTH
Stewart