1. Get 10-20lbs and see what it looks like in the tank. Add more or use less as needed to appeal to your tastes, and aquascaping needs.
2. I use a Maxijet 400 with 2 HOB filters for flow....if i had to do it again i'd go 1-2 sizes up on the powerhead. Or with 2 MJ400.
3. If i had to do it again i'd either go bare bottom or go with very fine white sand. Substrates are a pain to keep clean and an eye sore once they get dirty IMO. But take into account your stock. Your gobies might prefer substrate over BB, etc.
4. Coral doesn't add to bioload, stock away in that department. I keep 2 fish in my 10gal sucessfully, and until recently a shrimp as well. Makes an enjoyable tank. All the coral really give the color though. Go here and see my tank and the color, don't mind the algae.
My 10gal Nano (lots of pics)
5. Should be fine, just make sure the saltwater stays fresh and at the right saltinity level. As long as the rock is wet with good saltwater it will be fine and living. Keep the flow going in the tank during this time too.