A Squamosa would also be a good choice for a first clam.
I would position him directly under the MH light. Clams (especially young ones) need to be fed also. I reccomend DT's phytoplankton.
The Croceas are perhaps the most beautiful, and require the most light. Maximas also need more light than Squamosas, although the gold ones require slightly less than the blues.
With an 18" Deep tank under a 150 MH, the Squamosa would probably be a great starter clam. 3" or larger would be less demanding on the feeding than the small ones.
Have fun with it- but they ARE addictive!