Derasas should be on the sand bed because unlike Croceas and maximas (rock boring clams) Derasas actually lose the threads as an adult and rely on their weight to stay put in the sand.
Derasas are lower light creatures and like most SPS corals feed where light is inefficient, or/and use both sources for nutrients.
It's not exactly that Maximas are harder they just require a bit more resources than the lower light clams. If I were to name a harder clam amoungst the crowd it would prob be the Crocea only because the slightly higher vunerability to parasites due to their larger abyssal opening (below the clam).
As for lighting having a halide (be it DE or SE) and 20 inches of depth you could have a Maxima on the floor and it should be fine. The only time you'll need to feed the clam is when it's surface area is not large enough to hold enough zooanthallaxe to sustain itself. On average at 3-4 inches the clam can solely sustain itself via light (usually, it all depends on how much light you have for it). Before that it's nice to have phyto to feed it, live being the better choice.
As to the number of clams it's no so much how much light you have but conditions of your tank and calcium supplementation. Clams obviously can't grow without calcium, so if you have little calcium you have little stunted clams.
HTH
Hey i got another question though, now that my clam it attatched to the rock how do i move it. I don't want to move it anytime soon, but it is going to get way to big for my tank eventually..........
