There shouldn't be a big difference in rolling diameter if you're using the correct tyres.
I'd say that they're illegal. I remember someone saying something about if you've got 225mm tyres all round, you can't go larger in the rear and not the fronts. They must stay the same width (all four 235, 205, etc), but if they came with 255's in the rear and 235's in the front, then you can go what ever you like in the rear and front (doesn't matter if they're not the same). I would say that it's the same for rim diameter. If your car came with 16's in the rear and 15's in the front then it would be legal. I'm not saying that this is the law, it could be complete bullshit (I think it is), just thinking out loud.