You definitely want fusible interfacing, and I've heard more than one school of thought on stretchy versus woven. One person I asked (who'd made several T-shirt quilts) said that she used whichever was cheaper that day. ;) If you use woven, it will stabilize the knit fabrics. If you use knit interfacing, put it so that the direction of the stretch is perpendicular to the direction of the T-shirt stretch, and that will help to stabilize it.