im running cossie 4x4 brakes up the front and the fiesta center rear set up on the back all on the standard servo and master cylinder, and it copes fine with no problems at all.

as for the ZS rear beam, im running the standard frst one (as me car is a FRST ) but people say its stiffer and it handles better, but cant say from my own point as ive not done it.
the focus rear brakes, well the stub axle is different as the caliper carrier is built onto it so you change the whole thing, where as the fiesta center one you use your own stub axle, adn you use the hub, caliper bracket etc when i got mine, to save fucking about i got for 200and something, the kit, which contains, hubs, bearings, caliper brackets, bolts, handbrack cable spracers and wheel studs which are not fitted to the hub so they need pressing in.
on to of that i got a set of pads and some disc, which were standard solid and being a engieer by trade i knew i could drill them so i did, not that it needs it but it didnt cost me anything, and before anyone says, it does not weaken the disc and its is safe, as long as you know the right way to drill cast iron (which is slow and NO lube!)

you can get calipers fron fiesta center but i wouldnt, get ya ass up scrappy and get some, sierra/granda rear with sold disc is what ya need