Yes you can use your freshwater aquarium decor for saltwater (most aquarium decor is suitible for saltwater according to the labels when i browse through the decor isle at my lfs
If you browse through the saltwater showcase section of this forum, then you will rarely find people using fake decor in their tank. It looks very fake IMO
, but nobody's opinion on the looks of your tank matters except yours.
If you are trying to use a deep sandbed for biological filtration, then im not sure how your gravel will affect the effectiveness of it. If you are going foe aesthetics, then i suggest taking out all the gravel and adding a thin layer of aragonite sand 1-2 inches.
I sure hope you have more questions.