Scott-
Clams will benefit the most from metal halide lighting. Especially the blue maxima's and crocea's. That being said, you should be able to keep any clam under your VHO's, provided that you keep them higher up in your tank, and you supplement feed with phyto.
If you are talking Derasa's or Squamosa's, you should be able to easily keep them under your VHO's.
Good luck! I love clams - they really add quite a bit to a reef tank, IMO.
Edited by ssilcox