hey
You shall have no problem, running porsche rear disks and stock vw front disks, i am currently running that setup on my daily driven bug and have experienced no problems. When i fitted them though just to be sure i put the car oon a rolling road to test the brake bias and they were about the same, not idea but suitable for use. I personally prefer more rear bias as there is more of the bug weight at the back, jsut like on a porsche.
hope thats of some help
as for fitting, have a look here
www.geocities.com/meteorbug and check the porsche rear disk section