I agree with essop on this, Carpets are not an easy creature to care for, plus they get HUGE and dangerous. Nothing compares to a giant sarcophyton IMO and they can be fragged if they get to big. I have an ORA green polyp Sarco that has grown over 6 inches wide in the past 6 months, thats just scary fast. As for your clam, if you do decide on say a maxima or crocea be sure you place them in an adequate position in the tank. My little maxima did just fine until my tang decided it was her favorite toy, and my crocea is getting HUGE! It's about halfway up in the tank and is a happy clam, it's bigger than the crocea in the display at the shop now, thats what a steady suply of calcium does I guess lol.
If you do a clam be sure to stay on top of the Ca and Alk in your system.