Derasa clams are easy, they typically do not attach and work great in the sand.....
Crocea clams need to attach to rock, and were easy too, until I tried to move them and injured the foot.......downhill fast.
But they all will suck the Ca/alk out of your water. What do you have for lighting, they can adapt to alot of lighting, but obviously the more the marrier.
What's small, I have a 7 or 8" Derasa that I can part with.