Just an additional thought: Not all the shirts I got from my MIL were 100% cotton, but they were all a similar weight. Because they had all been washed (many times!) there was no shrinkage and they "played well together" in the quilt. I won't be so concerned in the future if I include a poly or a blend in a "shirt" quilt. It does work. Just be sure they are washable. I might even consider using a washable wool if it's the right weight.
Hmm, how about a crazy quilt with shirt fabrics of heavy flannel, washable wool, cottons, polyesters, etc? That might be fun! Well, maybe if I ever get all the other ones IMM (in my mind) done, that is!