Yeh I would. The 235 mps4s I'd say are as wide as the 255 pirelli's. I'm going off the fact that the 255 mp4s were dramatically larger than the 255 pzero's which were originally on the car when I bought it + what other people say about the 235's. I was actually amazed at how much wider they were. I know what you mean though, on my R I would always just replace the front 2, then all 4 and repeat since the rears lasted pretty much double the time.
No harm as long as the tread depth isn't insanely different I imagine. Some people, myself included experienced traction issues when changing the tyres where the light will come on when you hammer it for what appears like no reason but one click of the tc (putting it into sport mode, not completely off) solves this 99% of the time. My pfl was shocking at this, fl better but still apparent.