I have quilted several t-shirt quilts for customers. (I quilt on a Gammill Classic machine) My customers used a very light weight iron on stablizer on the backs of the t-shirts before cutting them in blocks. Most of my customers used cotton fabric for sashings to lower the bulk of the seams which made it very easy for me to quilt over the seams. I used Hobbs 80/20 batting and A@E Perma Core thread, which is a poly wrapped poly thread. This thread is a strong thread which has no breakage issues for me. Most of these quilts I have quilted free motion, but there have been some that the customer wanted a pantograph pattern used. I had no issue quilting with the pantographs. They stitched through the "painted" areas on the shirts just fine. The main thing that I watched for was to avoid needing to rip out stitches on the "painted" areas - as the needle holes will not always fill in and they will show.
Relax, have fun!