Thats good advice, Quattro cars with a Torsen centre diff as your car has should run the same size tyres front and back.
You could have different wheel and tyres front and back but the circumference needs to be the same or as close as possible.
In the manual it says something like " the tyres all need to be the same size, pattern and make on all 4 wheels" .
This is because some makes of tyre even though they are badged a certain size will be slightly larger or smaller.
You are best sticking to one wheel and tyre size.
Karl.