I wouldn't serge around the edges as I think that will make the seams too bulky which will create more problems for you with the hand quilting..just get the lightest interfacing you can find and I think you will be happier with the results using it on the entire back of the t shirts...I hope they are paying you lots and lots of money for this as machine quilting these are complicated enough. I just longarmed my own tshirt quilt and my daughters baby tshirt quilt with a minky back with really nice results
