ball point needles are needles specifically made for sewing on knit fabrics (like t-shirt fabric) they have a ball-point instead of a sharp point- move between the knit instead of cutting the threads.
also the type of interfacing you used would have something to do with any distortion after the quilt is finished....the interfacing should keep it smooth/flat without any wrinkling, the knits should not really shrink or distort. you might want to take a piece of (scrap) t-shirt fabric & make a practice sandwich and do some experimenting to see what stitch on your machine or design you like before actually doing anything on the real quilt pictures, so you don't do something you don't like then have to take it out.