one thing to try is nylon webbing and clips(for easy removal) to connect the thy pieces to the rest of the armor. it should help keep the thy pieces from sliding down on to your leg and onto the shin/knee pads.
in the pepakura program you can change the paper size and re position the parts so that they fit on the smaller or larger paper. if a part is to big you can cut it in the program or you can have it cross multiple pages and just tape the part together when you are cutting the templates out. PS...