If you're diligent about tank conditions I believe a Maxima could thrive in your tank. I'd suggest starting with one at least 3 inches long and place it centered under the halide.
I don't understand the concern about Nitrate by the previous posters. Clams thrive with Nitrate. In fact, adding a Tridacna clam will definately lower your nitrates as they make very good filters.
There are a few concerns about small tanks.
1 - The Clam may outgrow it.
2 - Stability of water parameters, temp, salinity, PH, etc.
3 - Small clams require more phytoplankton than you can reasonably add to a small tank.