1. Do NOT, repeat NOT bother with putting rear discs on. Will it improve the braking? Yes. Will it be worth it for you now? NO. Not at all. If you want a solid braking improvement, put on 10" front brakes from a 16V Scirocco (you can use early Audi 4000 calipers too). I have owned more Mk1 Chassis cars that I can remember, and some have been 4whl disc, and some haven't The best useable street brakes were actually 10" front discs and larger rear drums (pickup or Fox wagon).
2. You canNOT, repeat NOT put the front brakes on the rear. If you were a fabricator and wanted to make a completely custom setup, it could be done of course, but there is no way it can possibly just bolt onto the rear. Trust me - I researched into it heavily in the 90s when I was researching for ways to get around certain rules in SCCA. There was a factory race car that used the front rotors in the rear, but a custom caliper adapter and caliper and NO e-brake. Which is suxor on the street.