Honestly? Clams are quite easy to take care of, they don't require feeding, once in place don't move around and are beautiful to look at.
However.... in order to make them easy to take care of you need:
The proper lighting (which you do)
Calcium of 400-450 ppm for them to grow
Clean water (0 phosphates, nitrates, etc)
NO direct water movment
As far as placement, I have a crocea, deresa in the sand and moved my maxima from the sand to the rock just based on preferance. I do agree with trying to get one over 3 inches, but not mandatory as the maxima's seem impossible to find that big. I do also dose iodine, trace elements, and stronium once a week for everythign that lives in the tank.
Good luck with your clams, if your tank is set up and your water parameters are fine you shouldn't have problems.