In my opinion having the Culture and Chaos in the same universe would be like having a cold fire or silent lightning. WH40K cannot stomach a benevolent utopia in the same way The Culture universe (as in, our reality) cannot contain magic. Had the Culture formed it would've done so in opposition to this Manichean Satanic force in its long, long history. Unless Chaos is specific to the Milky Way Galaxy, in which case The Culture shouldn't bother coming here in the first place.

Saying though "how would the culture deal with chaos" -- would presume it hadn't already.