For the T-shirts I would want to stabalize them prior to cutting the pieces to put in the block. Follow the manufacturers directions to apply it. Since you are using heavier fabrics, you want them to be larger pieces so as the seams allowances from these fabrics will be bulkier. Also may consider placement with these fabrics and not put them next to each other. I am sure it will turn out to be a treasured quilt for the family.