I had always heard the 50/50 rule you can run a flat RFT up to 50MPH up to 50 miles and from what I've heard more than a few miles you start counting down the clock and when you get to 50+ your buying a new tire. I carry a slime repair kit with a tire pump and a tire plug kit figure the price I saved on tires more than exceeds the towing expense and another tire if something ever happens. As for non matching tires it's not a big deal if they are comparible tires and since the RFT's don't do anything but run flat and rain decent then the fronts are probably better so there should be very little concern other than rain and the first time you drive in it you'll know how it is. As quick as these cars go through tires I would just wear then out and then replace them.
2008 135i - Cobb/JB4, Spec 3+ Clutch/Flywheel, MS DP's, Berk street exhaust, AMS IC, UI CAI, CDV delete, Quaife LSD, M3 control arms/Links, RE Toe Link, ECS Trailing Arm, M3 rear SF & Trans bushings, SS brake lines, Michelin PSS.