Not a good idea to mix run flats on 1 axle and non run flats on another axle. It will throw off the suspension geometry and the car may do some unexpected things since the front run flats will not allow the side wall to roll at all and the rears may roll. All in all not a great idea. I'm running non run flats now but on all 4 corners. I'm noticing the car is slopier and that is due to the fact that the stock dampening is tuned for the very stiff run flats! So like most people I will eventually be upgrading my suspension with new dampners and m3 roll bar.
Hope that helps