If they are similar in weight to "ordinary woven quilting cotton fabric"- then I would use them as is.
If the shirts look very worn/thin - and you need to scrounge for every last bit of material - then I would use a thin fusible interfacing on those fabrics. I would also give the shirts a bit of a "yank test" to see how durable the fabrics still are.
I don't know what condition these shirts are in. Some "old/worn" shirts still have usable parts on the parts that are below chest level.