My center has a mix of batik and cotton in it. I've always mixed them and never had a problem, sometimes those colors just work out that way.
For those that haven't been in an RR before...it's much easier if your center is divisible by an even or odd number. Example 18" center is divisible by 3 or 6, a 15" center is not. When you start adding the rounds it's much easier to figure out a pattern for each one that comes out even on the sides instead of an inch left. Seems like there is lots of odd shaped corner stones on those odd sized centers. Just a suggestion, learned from doing other RR's.
I'll take a pic later and try to post, haven't had to much luck at posting pics after they changed the format on here. I'll add some extra focus fabric if anyone wants to use it and a little message.