I believe most clams do require strong lighting. You certainly have plenty of room in a 55 to include additional fish beyond 2 clowns. Many gobies are easy care, and are kind of fun to watch as they spend a lot of time scooching across the substrate. You should also plan for a "clean up crew" to help
control algae on the glass and rocks as well as
detritus in the sand. Examples would be hermit crabs, turbo snails, nassarius snails (good to keep substrate oxygenated too), and various starfish (some of which are also good at helping keep the substrate oxygenated. Feather dusters are also very cool and relatively easy to care for.