I just ran across this from another thread and remembered one of your questions.
Hi Clode, welcome to the mixed media forum Brian asked me off-list about how to keep his images from wrinkling. I told him about the method of applying a thin coat of acrylic medium to both the front and back of the image, letting each side dry. Applying it to one side will cause it to curve and wrinkle, but when you apply it to the second side it straightens back out. This is a method developed by Jonathan Talbot, and is very helpful with delicate images and papers.
I work a lot with thicker papers so usually don't have too much of a winkling problem. Handmade papers, ect.
As far as your ribbons, I have found that they and even some papers, will deepen their color even after drying. If I'm in doubt how a piece will look afterwords, I'll do a small test piece first and keep the finished color in mind.
I feel you can use just about anything in a collage, you just need to adapt it's "personality" to how it will work into your piece.