You will get different answers from different people. All I can do is tell you my experience. I also have PC lighting. I started with a single light as you have now, and I have kept a couple of croceas successfully for almost a year now with nice growth. About 4 months back, I decided I wanted to add some additional light, but not yet ready to spend the extra money on MH, so I simply purchased another PC, giving me a total of 400w over the tank. I currently have a 6 inch maxima, a 4 inch squamosa, and 2 croceas (one small, and one about 4 inches). They are all doing well and showing signs of growth. I also keep several different SPS which are also not supposed to grow in my tank, but they do, and they do well. I think that as far as the clams go, placement is very important as croceas and maximas like and need to be closer to the light and therefore not left in the substrate of the tank -- which is where the squamosa seems to prefer it. All I can share with you is my experience. The only other thing I can say to you is that, as always, what works for me or someone else, may not necessarily work for you as nothing is universal. I say, as long as you are informed, go for it. Best of luck.