Yes, I have mixed buildings from many many sources and been quite happy. But frankly the K-Line Buildings - at least the Mainline America line I'm thinking about - are a bit unique as to look: a tiny bit toylike, but detailed and well made, not completely"toy." I have a copy of the 2005 catalog -- its the only place I've ever seen some of them and they are jewels - just beautiful.
I suppose a few Lionel or Dept 56 buildings would fit in among them. I don't know if repainting Christmas buildings over the white snow is feasible.
I think I might be very tempted to hold back and try, over time, to do all_K-Line. I would be a unique layout: I've never seen a layout with lots of buildings and all K-Line. At most, you could add Lionel and other buildings on a temporary basis, knowing it would take time to find/buy all K-line. I'm not sure how many different buildings K-Line made, but if not enough you could carefully repaint second copies of some to give them a different look. Valley Toys and Games, for example, could become a believeable and attractive corner drug store . . . etc.
Good luck. If you have pictures I love to see what you already have . . .