I take a little extra care introducing a breastcollar and flank cinch. I've never started a baby from the ground up, but I've never purchased a horse that I knew, beyond a shadow of a doubt, was totally familiar with full western tack.
I would just make double-sure that the horse is very accepting of my hands rubbing the area where the equipment will sit, and of the tack in place and correctly adjusted at a standstill and on the lunge at all three gaits before mounting up. As far as a complete greenie, I think I would go the first ride or two with saddle only (the experience is going to be unfamiliar enough), and then add the "extras". Of course, that would just be my personal preference - there isn't any hard and fast reason why doing otherwise would be a bad idea. If you've done your homework, there shouldn't be any problem.
A stubborn horse walks behind you, an impatient one in front of you, but a noble companion walks beside you ~ Unknown