We discussed the sand bed issue. Forgot who, but someone quoted an article that was talking about th studies of DSB and how in reality most of the bacteria works in the first inch of sand. It was an older article though (1980's) and I suspect more recent work has proven that study wrong. It just doesn't make sense to me that aerobic and anearobic bacteria can live side by side in your sand bed.
Mike knows his tank, so I'm not sure how he is doing it but I trust he is. There might be some other variable allowing for anearobic bacteria in his more shallow sand bed. Bornemann, Calfo and Fenner all seem to be in agreement about 4+ inches, however, so I'm going to stick with them and maintain my bed accordingly.
Now, to the poster's questions :happyfish
Your sales person is wrong with just about everything you stated they said. Do some research on this site, then go buy "A Conscientious Marine Aquarist" by Fenner, and other books by referenced authors. It's really best if you read for yourself and ask questions based on what you have read. It will help you distinguish between a good salesman and a intelligent salesman.
Normally sand is cheaper than "live" sand. So people often buy just enough live sand to place over the top to "seed" their sand bed.
You shouldn't put live rock on top of the sand. Put your lowest level of rock down first, then put sand over it. That way your base to your upper live rock is stable. because it is placed on rock. Fish and inverts can otherwise burrow under your rock and casue a landslide.
Some corals and anemones need to be placed on a sand bed. That's where they are found in the wild...
Add the sand now. also, do a search on this forum for "cycling" your tank. The easiest way to cycle a tank is to either use uncured live rock (easily purchased online) or to throw a raw, cocktail shrimp into the tank to let it dissolve. Either way, the decomposition will start your ammonia cycle.
Anemones are delicate. A tank needs to be well established in order to keep them properly (most say 6+months). Two great books to get to aid you in purchases are :Michael's "Reef fishes, a handbook...) and the similar book (forgot author) for corals.
There is a pretty steep learning curve to this hobby (made the more difficult by anaware or unethical local fish stores) but one day you'll walk into your tank's room and suddenly go "Woah.. my tank looks great!"
Keep learning and keep asking questions!