to answer your question, it would help to cycle the tank, and i too woudl only use the ls,a nd completly forget the cc, eventually i t may be a problem, if yo are cycling your tank now, it is a very good time to make the change,yu probably will in the future anyway, so now would be best just because you have no fish in it
i personally would have a 4 ot 6 inch sand bed and make it mostly of dry sand and jsut one or 2 basgs of ls, the dry will sooon become live
even if the lfs says to use cc, it is not the best thing to use in a sw aquarium(in fact, it is not even recommended by hobbyists anymore)