For this quilt, I would use a poly so there isn't a shrink question. Your did not make an 'oopsy' with the darker fabric - you added a design element! Trust me, we have ALL made changes midway thru a quilt to work in our unexpected design elements. The absolute most important lesson is to keep having fun. The biggest cause of UFOs is that they stopped being fun and became work, so we put them to the side.