New Tank Setup

Hi, i am setting up a new marine tank; here's what i got so far :

55 gal tank
fluval 404 filter
CaribSea Reef Sand
I started to cycle the tank with CoralLife's Nitroplus and Sal****er
Nitrigying Bacteria
No fish until i am sure my water quality level is good enough to handle
fish, don't want to see my fish to die

I am planning to get live rocks but when i was in the store, i asked the
owner and he gave me some interesting information. he said i don't need any
substrate for a saltware tank ( which i have bought already ), he suggested
live rocks.
Then i went to another store, the store owner gave me a totally different
story, he said i need at least 4 inches of sand ( and no crushed coral ),
according to my research, crushed coral is needed for the alkalinity.

As most of you are, i started with freshwater and this is my first attempt
on sal****er.....even with all the research and reading, i still get
conflicting information...a bit frustrated?

So here are my question, why would adding live rocks will speed up the
cycling, does it generate ammonia or bateria? i was planning on buying them
piece by piece to slowly build up my tank ( yes ... because of budget ).
Do i need substrate and why?
What brand and kind of protein skimmer is recommended?

any help is appreciated.