You probably can't go all the way down to 15" rims, but you should be able to go smaller than stock Crossfire OEM rims.

The stock rims on the SLK320 were 16", so that should be safe on a NA Crossfire.

Be sure to find a set with similar overall tire diameter for both the front and back to either the OEM summer or OEM winter reccomendations.

(I think the smallest rims that will clear the front brakes on an SRT6 are 17" rims.)