Lots of BMWs come standard with different size (width) front and rear wheels.
You can look up the stock 18" Ci wheel and tire sizes on the BMWUSA web site. I am sure going with that configuration would work fine.
I do think though that BMW tunes their suspensions to match the stock wheel and tire sizes and performance, so I wonder if all the Ci suspensions are tuned the same even tho they offer different wheels and tires as options? For example, is a 330Ci w/18" style 71's tuned the same as a 330Ci w/17" style 68M's???