You can mix the fabrics as long as you can launder them together. You will need to use an iron on stabilizer for the silky fabrics. Be careful and choose one that you can apply with a heat setting low enough that won't melt the silky fabrics, which I'm assuming are synthetics. As far as the backing is concerned, the web site you linked to doesn't use a backing fabric. Looks like they just applied the squares to a blanket. If you want to make a more traditional quilt, using batting & backing instead of the blanket, you need to provide the estimated size in inches for length and width in order to calculate yardage.