[I think you said it already. A clam will require at least a 250 watt metal hallide, depending on the clam & tank. A deresa, or other less photosynthetic clam, might be able to survive for a while under a 150 watt MH, but the point in keeping them, of course, is the amazing color. Even that wouldn't be a good idea for more than a week. They'd turn completely brown in a very short time and expell their symbiotic algea.
As long as you don't get one from some of the places in California that have that coughing disease that might kill other clams, there is no point in quarantine.]