Another thing to consider....
Although the decorations are beautiful now, they will be very ugly soon. The natural process of a sw
tank aging will bring things like diatoms early, algeas later on, various other things that look good on natural rock. Things will grow and it won't look pretty.
I'm wondering if the fish store has offered to come maintain your tank for you for a price. I'd get a good book, do some more reading, and consider not adding a thing until you know more without relying on the fish store folks.
What size is the tank and what kind of filtration?
Also, the heni and hippo tang are really sensitive fish for a beginner and a new tank. Who's eating? Are the damsels any where near agressive acting towards the new fish. Often, they can be mean little buggers and cause problems for even bigger fish later on. That can mean stress and sickness.
What kinda test kits are you using?