I use DMC because it is what most plates refer to colorwise. Some will give alternative numbers for other floss. I smock with 3 strands, picture smock with 4( design inst. should tell you). You stitch in the top1/3 of the pleat. In the space between picture smocking you backsmock on the back of panel in a matching color. As far as how much it stretches-you draw pleats up and block(steam)them before smocking. When you remove pleating thread it will stretch some but should not be all stretched out of shape. I remove my pleating threads after I complete the garment. If you have any other?? please PM me and I will try to help.