An upcoming project and a question

Hi all! This is my first post to this KAL but I have been lurking in the shadows for a while now, being very impressed with all the beautiful projects you guys are making. I received the book a couple of days ago and am now waiting for the yarn for my first project, the Perfect Periwinkle Vest, to arrive. I can't wait to get started!

In the meantime, I have a question for you all. I would really like to do the Tweedy suit in the back of the book but I think that the bulky/chunky yarn would make it too warm. Has anyone out there tried their hand at this project? Any luck/unluck with substituting thinner/another yarn for the one called for? And how do you think that a thinner yarn (say, dk) would affect the drape and shape of the garments? Redoing the patterns for a different gauge doesn't frighten me as much as knitting a garment that will look totally different from how I want it to look like! Any comments, suggestions and advice are more than welcome!