There is a first time for everything!
We got our first pair of socks to add to a T Shirt quilt. No problem- it quilted great!
We do get sweaters sometimes and they cannot be quilted. The quilting machine will not go over the thick yarn. We can still use them but we have to quilt around them. We then go back and tack the sweater down so it will not sag.
We discourage using sweaters unless the T Shirt quilt is for show only and you don’t plan to wash it a lot! Frequent washing will cause the sweater block to distort.
The t-shirt quilts we make are densely quilted to hold their shape for many years.