That's fantastic! I'm so glad! It will be a lot of fun having you along, and no worries about questions. This is sewing by the seat of our pants, and figuring it out and designing it to suit our tastes and ideas as we go along.
You can pick whatever color of fabric you want as long as it's 25 count, although you could do a different thread count, if you wanted to. I can't say with any certainty, but I think the main thing that would be affected would be the lettering for the cover.
Regarding colors, I'd pick colors and threads that appeal to you - without regard to what the others are doing. When we were working on the Nesting Place, it was really fun to see how Mirabilys introduced different colors into her book than those originally specified. It got several of us thinking about trying out some different things on our books so that in the end, they were all different.
So use some threads from your stash, or get some that you have had a hankering to try! It is Needle Play after all!
Also, I have a feeling I might try to incorporate some of Mary's little stitching projects into those squares. Wouldn't that be fun?
I'm glad you're joining us.
If you use a fat quarter size, I think a 14", 15" or 16" frame or combination of those lengths will make finishing easier at the end.