I wouldn't reccomend using front brakes on the rears, you need to try and keep the braking proportions with front and rear at a safe level. You would have as much braking force on the rear as the front, & your car wouldn't be stable, so you'd need a bias valve. You'd also need to fit a hydraulic handbrake as there would be no locking mechanism on the brakes. And you would probably need a bigger master cylinder as the small puma one might not cope with the capacity.
You can fit the rear stub axles from a mk1 focus, although you'll need to redrill for abs sensors. Also the ST150 rears are the same. The ST170 holds the caliper slightly further away from the centre of the disc allowing bigger discs to fit. But all the rear calipers as the same on focus, ST150, ST170, and FRP.
Only other thing to change is handbrake cable, FRP one fits best, but most use the focus cable.