You have to use their fabric and their backing fabric. Apparently some people were substituting lesser quality backing fabrics. The new kits ask you to send back the packing slip that lists your fabrics.
As to design, I think you can do it different. Just watch the amount of fabric you need-my last kit had fabric that was cut crooked and raveled (is that the right word?) when you handled it. I wouldn't want you to run out of fabric.
You got a really cute kit!!