Good questions Melissa! I'll try to answer a few of them....
As for the BOM we have here, it actually started as one-a-month, but so many people signed up to pick a block pattern that we opted to post 2 per month so that it wouldn't be years long! You can join in anytime, and there is no pressure to do ALL of the blocks - you can just make them as you can, and if you think one is too far out of your comfort zone, you can skip to the next one. You pick your own colors & use your own fabrics.
I think each of our exchanges has its own set of "rules", set by the person who organizes it.
Hope that helps :D