SuzzyQ

You can also use the back of the t-shirt. Nothing says it has to be printed. And you can use the lightest Non-Woven fusible interfacing to back the t-shirts and keep them from stretching. No need to use interfacing for knits. The idea is to keep the knit from stretching.
And don't ever quilt through the printed part of the t-shirt, especially if it is iron one; printed is okay.
And yes you can mix in quilting fabrics.