ill start this off....
1. theirs many factors that contribute to how long a fish stays in qt. type of fish, method of qt, chemicals being used, etc.
2. i wouldnt put rock or sand normally, but if you quarantining a wrasse that hides in sand i would make an exception. side note: once you finish qting that wrasse you have to throw out the sand.
3. i would make a seperate qt for fish and coral. coral are highly sensitive to copper and other chemicals so i wouldnt qt together.
this is just some general info. theres plenty of older threads you can look up to get into more specifics.