The answers to your question about editors and stylists (and the difference) there's some good info in this thread ...
Jobs/Positions at Fashion Magazines.
Don't think that there's info about the pay scale for in house fashion editors/stylists ... but I'm pretty sure it depends on the magazine. The more important the magazine ... the better the pay and the more responsibilty you would have.
A freelance stylist can work for magazines but it's just here and there and there's not much pay ... since it's for marketing purposes, mainly. Most freelance stylists in the print world do catalog and ad work and are paid by the designers ... not magazines. See the Stylist's thread for info about that and how to get into freelancing.
You can't moonlight as a freelance stylist while holding down a full time job ... unless you are working at night or can just not show up on your regular job whenever and even that could be a conflict at times. Most gigs are during regular working hours.
You can work up through the magazine hierarchy if you want to be an in-house stylist/fashion editor (same thing, in most cases) at a magazine. Intern at first (if possible), then assistant to an editor, work in the styling closet, even the mail room, things like that. You learn by being there and you get promoted. Pay would be minimal the first few years, of course.
I don't think that the FIT certificate in styling is very important to a magazine nor will it help much in learning skills for the job. Magazine work is different ... only having the eye for style is the same. I beleive the course is geared to give you some of the basics of freelance styling to start your own business, to learn how to source, about pull letters, building relationships, doing your paperwork, ... all freelance skills you need to learn. It won't hurt to learn it ... but it won't be a lot of help and I don't think it will impress anyone hiring at the magazine.