You have enough light for any clam you prefer and you are fine to add a clam at this point. People put too much worry in owning clams they are alot more tuff than they are given credit for. I have a 34 cube 165watt Reef Radiance fixture and 7 clams 2 Squamosa, 1 Derasa, and 4 Crocea. My biggest clam is bigger than a 20oz drink bottle. I've set clams in newly setup aquariums and they do fine they absorb ammonia and use it for food.