If you're doing a rag quilt, you won't want to apply any interfacing to the sweatshirt. If you do, it will show & I don't really see the purpose since you want a rag quilt to unravel a bit (although I agree with Barb that sweatshirts usually have little to no unraveling). The other thing I would do is slightly (0.2mm) increase my stitch length on those seams to help accommodate the extra thickness of the sweatshirt fleece.