Hi Madbury - -
So glad you posted your questions! The second fabric in the 4-patch should be white or a white tone-on-tone. You may use a different 30's fabric for each of your quilt blocks if you like. Yes, it depends on how many swappers sign up whether or not you will get any of your blocks back.
Here's a picture:
[ATTACH=CONFIG]446617[/ATTACH]
I just chose 30's to get things started. What other kinds of themes sound like fun?
Thanks!
Jane