Hi,
DT's dosing instructions are on the back of the bottle. Just dump it in after lights out, you can leave the skimmer on.
BUT, clams over 3" really don't need all that much "food", at that size they mainly use light as they become photosynthyically capable. As for placing him on rocks, yes he will enjoy being on the rocks but many clams seem to hop around and fall off. I would get a oyster shell and take half of it and place the clam on the shell. This will allow a place for his foot to attach but he won't be able to hop around as much.
As for the elegance, those are hard corals to keep anyway, not to mention you clowns took too it right off the bat. This more than likely stressed the heck out of it.
~ Ok I just went back and looked harder at the pic. You have a lot of softies and LPS right together, maybe even touching in places. FOR SURE the LPS would take care of the softies, they have decent sized stinging / feeding tentacles that reach out at night. So they can kill from a far. Actually I am suprised that the LPS didn't suffer as well, mushrooms are anemones and have the same stinging capabilites of an anemone. So you bacially have 2 of the best chemical warfare and killing sp. right there together.
Sorry for your losses!