if you're replacing the floors because they are rusty, either Tremclad (or Rustoleum as you call it in the US) would be fine, POR15 is intened to go over rust. As for rockers, you should really be replacing inner and outer if either is rusty and that's the reason for replacing... also, when you do the quarters, I assume that's due to rust either around the wheel lip or behind the rear wheel housing opening, in which case the inner wheel housing probably has a hole where water has gotten in and rotted the quarter panel out. In the case of the quarter panel, what I would do is cut out the damaged parts, strip the rest to make sure it's ok (then epoxy primer it) replace inner and outer wheel housings, then weld the quarter panel skin over top of the old one, this way you dont have to worry about flexing or anything like that. The car should be sitting on the suspension whenever you're aligning panels especially if they are welded on so they are lined up properly.