A few questions for a newbie at saltwater fish tanks.?

[Bill Boller]

First can I add inverts with fish, I know there are some fish that eat them but generally is this ok to do? Second how many fish can I add at one time and how long do I have to wait before adding more. I heard 1 or 2 at a time but if they are more hardy smaller fish (such as damsels or clowns) can I add more at one time. I have a 55 gallon tank and have 40 lbs of live rock i was told this was ok but i have read I should have at least 1 lb per gallon. Is this true or because the rock and sand takes up space I don't need as much? Lastly I purchased 2 damsel fish after a week of having the aquarium (I was told they would help with the cycle) a yellow tail blue damsel and a Black-Tailed Dascyllus (or it is a 3 strip damsel I can't really tell) and the blue damsel is great! The other one though will just hide and he has only eaten once in 3 days he normally just hangs out in a cove and only came out once for food. Should I be worried or is this normal, I have read they are shy and need hiding places or they will be aggressive but i would like him to eat. Thanks for reading this I know its a lot but I need help!