If all the fabrics have been shrunk, it would probably be okay to mix flannel and "ordinary" quilting cotton.
I prefer not to mix batiks - because they seem to be so tightly woven as compared to the ordinary quilting cottons - with the "regular" quilting cottons.
I also prefer to keep approximately the same weight fabrics on the same side - for example, I would consider a denim backing with lighter weight fabrics for the topping.
There really are no LAWS about what you can or can not do. It seems to come down to what you like, what you have available, and can you make it work out.