well crocea and maxima's don't get too huge. If you have a squamosa, or derasa you better plan on haveing a large tank, 125 g min. Or have plans of getting rid of it when it gets bigger. Giga's require even bigger tanks, they get up to 30" in some cases. Snails know better than to get too close. Some fish aren't too smart though, if a clam closes down on a fish both will probably perish. Clams are ok with most other reef safe inhabitants. Some tangs, blennies, shrimp, most angels, and wrasses aren't clam safe.
You have to have perfecly stable water paramaters and good lighting to keep clams, they are alot like keeping sps's.