Well, if the wheel shop knows what they're doing, then I say 8"/8.5" is safe for stock or modestly widened fenders.
If I were you, I'd look for a pair of front wheels and use them at the rear. That way you don't have to chance screwing up a good set of wheels.
www.wheelenhancement.com can partially trade your wheels for the 7.5's.
Edit: Just noticed you're in Greece. You can probably forget Wheel Enhancement. I'd still look for a pair of front wheels, though.