Re: necessary to have diiferent size of tire
unless one does really aggressive driving, or, has tremendous power going to the rear wheels, or your rims size already comes staggered, dont go for staggered tyres setup. a useful point is that staggered tyres setup promote a more understeer inclination which is what most average drivers can safely handle.
however, it's always cheaper to buy 4 x same tyres, you can change front to back easily, and for those cars that carry full size tyres+rims, you can use it without screwing up the geometry of the car.
or, you can just go for staggered setup just for the shiok feeling. but it's not advised. The car always feels nimbler on similar sized tyres, compared to one with a staggered setup, coz the E46/E90 has a very well balanced chassis, so it's good to maintain the neutrality.