as all 4 pots use the same sized pistons it does not matter where you get them from but i would look at customer service and also the positions of the bleed nipples and the size of them as some are in the wrong place and are smaller than others.
rear cylinders you the same as you should for the front discs. then you will be having a set up suitable for the MC
again read the link i posted above as it covers the reason for grooves or drillings or both.
and re using an adjustable pedal box the sizing depends on what other bits you get but the rear of the car require so little braking power i feel the cost of the adjustable box is not really justified.