If you google "aesthetics of aquascaping paul whitby", you'll see 3 awesome Reef Magazine articles on aquascaping principles.
Wow, southdown play sand, do they sell it out here? I know it was big in the hobby 15 years ago, when I did my last build, but that it was only available at Home Depot on the east coast. I bought some when I lived in Boston. $3 per 50 lb bag, can't beat that!
Where did you get your rock, and was it live (or wet) when you got it? Should be fine to use by now if it is dry and doesn't smell bad or have dead stuff on it. Otherwise you should rewash it in bleach for 24 hours (fresh water plus added bleach, put powerhead in and let it sit overnight), then wash off good and redry.
You could also acid wash it, which some of the guys in the club have done (including me). That is a sure way to make sure it won't cause any problems for you.