If you aren’t sure if some of the shirts are all cotton, I’d stabilize those in particular. I recently made a t shirt quilt with cotton blocks in the corners. I stabilitised the t shirts but not the corners. It came out great. I used Pellon shapeflex SF 101. It’s a fusible woven stabilizer. I like and use it a lot. You can get it almost anywhere. Walmart Joanns and amazon.