I think you are right to add sashing in between the T shirts. Any good 100% cotton quilting fabric would work for the sashing, just pick a color or print that you think coordinates with the shirts. I would use just one fabric throughout since the shirts are probably different colors except you could use a second fabric if you wanted an outside border. Other threads on this board recommend not to try to quilt through the center or the lettering on the T shirts especially if they have that rubbery texture--just quilt around it. Good luck!