Hi. Based on your location, I'm guessing you're asking because that's what you have? If so, I'd say the Kölsch yeast should work just fine (I love a good Kölsch!) As for the Hallertau, again, it should be fine, but will definitely change the flavor character of the beer. In addition to BM's original version (which is excellent,) I've tweaked my other versions (I'm on my sixth batch now) by changing some of the base malts, hop additions (Ahtanum is a really good flavor/aroma hop addition,) and yeast. This is truly one of the great beers that shines just about any way you make it. I suspect with a Kölsch yeast and a noble hop, your version may come out more like a German Blonde than an American Blonde. Still wonderful, just with a little different accent!I wonder if substituting hallertau mitterlfruh for the 20 minute hop would change things too directly
Ed