Tom,
If you want to use Rejex, I'd recommend using the Collinite cleaner first. Rejex is one of those new fangled polymer coatings (insert marketing gobly **** here) that relies on a clean bonding surface to adhere to.
If you are going to use a traditional wax (I really liked the Collinite liquid wax prior to Rejex) you can probably get away with a quick cleaning of the hull using dish detergent.
Rejex can be used on any fiberglass surface, the hull, cabin, etc.
As far as the deck goes, some people swear by Woody Wax, but I found it very expensive and that it didn't really buy anything for me, even after following the directions to the nth degree.
I've just accepted the fact that the deck is going to get dirty and will need an occasional cleaning. In the past, I've used Soft Scrub with bleach to get out any stains and keep things clean. It works great, but I found that if I wasn't very careful about rinsing it down after, some places would get a chalky residue.. most obviously on the rub rail. You also have to be careful because it contains a small amount of abrasive powder.
About half-way through the season last year, I picked up a bottle of Starbrite's non-skid cleaner with Teflon and found that it did just as good of a job, without the residue or abrasion. It may have been in my mind, but it seemed like the teflon coating (more marketing stuff) helped prevent stains from 'sticking'.
My boat slip is several hundred yards down wind from a highway overpass, and tends to pick up a barely noticeable layer of black soot over time, and I think the Starbrite made it easier to just hose it off, rather than have to scrub it away with the Softscrub.
Do some searching here, there have been similar posts in the past. A search on "rejex" will probably bring up some good reading material on other cleaners/waxes as well.
-- Tom