I've had my squamosa clam in a 75g tank for some time now under 412 watts of PC lighting (2 x 55w actinics, 2 x 55w 67k, and 2 x 96w 10k) and he's been doing great.
It seems as though popular opinion on these forums prefer VHO's over PC's. So I guess it all depends on how much VHO lighting you're giving you squamosa. But like I said, my guy is doing great without MH's.