I recently got a flat rear and my welfare ass can't afford to replace all four tires right now (my rears are very worn, fronts are OK), especially not at 285/30/18 size. I'm currently running Potenza S-03s, but I'm having a very hard time finding them in my size so I'm considering getting RE050A PPs.

Is this a bad idea to run RE050A PPs in the rear and S-03s up front until I can replace those too?

 

Running 2 different tires is not recomended, but as long as you use caution while driving you should be fine. I am running stocks in the front and Azenis in the rear with no problems, but I wouldn't track the car with this set up

 

Those two tires you mention are also very similar handling tires. The RE050A Pole Position is replacing the S03 in the Bridgestone line.
If I can help let me know.