When a saddle tends to roll, it could mean that the saddle does not fit, you are riding unbalanced, girth is too loose, you have a very round, wide horse, et cetera. These things generally cannot be fixed with more or less padding.
Depending on the type of riding that you do, a thinner pad is generally a good idea. You want enough pad to, you know, pad the horse, but you do not want to be "perched" on top of the horse. If the saddle fits correctly, you do not need a thick pad. Again, it depends on what type of riding that you do. For western, I think that 1/2 inch is minimum and 1 inch is about the maximum. Those are just guidelines.
Do not fold over/double up unless you need a makeshift shim. This will only unbalance the saddle.
It is okay for the saddle pad to extend past the hip. The saddle pad is soft and flimsy. The horse will have no problem moving their hip under it.
Look for a short, round-skirted saddle pad. These tend to work with short/flat-backed horses.