I agree that it's all about the value and contrast.
But I also find that the more diverse and discordant the fabrics, the smaller the pieces should be. you can blend pretty much anything into a pleasant whole so long as the pieces are small enough and there is a lot of variety. If you have 98 % flour sack prints and 2 % batiks, those batiks will look out of place, but if you have just a huge mix of every color and style of print under the sun, it can work.