Hi Minpin, welcome to AA.
1. Can you elaborate? You mean it's filled right now with freshwater? Or is it all setup with decos and such? I would dump it all and start new. Do you have access to RO
water at your local grocery store? That would be ideal to fill with.
2. The beauty about all this is creating what YOU like best. Check out your sand options and pick one that you like the best. I will say that I would stay away from crushed coral and oolite sand. They are too big, and too fine, respectively, for my liking. Regular aragonite sand is best. I used aragonite sand first, then covered it with a layer or Arag-alive livesand. Regardless, your sand will become "live" over time.
3. Cured is full of beneficial bacteria, and like blazey said, it usually is void of critters. Uncured rock is not fully established with nitrifying bacteria, and usually comes with critters.
4. Reccomened if you want to do a reef tank. If you wish to do fish only, IMO
skimmers are not needed.
5. None at all. Just your sea salt.
6. I have a 29 gallon with 2 Ocellaris clownfish and a lawnmower blenny. I may consider adding in another nano fish, but that would be about the extent of the fish I would add. 3-4.
. And don't be afraid to ask!