If your saddle slides around, then it does not fit your horse. If it's not your horse, then you need to talk to your instructor about using a saddle that does fit him. If your instructor is not helpful, then I might suggest finding another instructor.
If the saddle slides around, then that usually means that the saddle is too tight at the withers. Adding another pad is not going to help. In fact, it will make it tighter, which will be more uncomfortable for your horse. Try riding without any pad at all. If that helps, then get the thinnest baby pad you can find and a Pro-Choice neoprene girth. Do not add extra pads.