First, any English saddle needs to be level. You should always place the saddle WITH pad too far forward and then pull back towards the tail. As soon as the pommel levels out, you have cleared the shoulders. Also, you should be able to fit four fingers between your horse's elbow and the girth. I ALWAYS try to have the same # of holes on both sides. If one side is higher, I make it the off side. Think of your own clothing. One side higher feels strange. I believe your horse feels this, too.
If your saddle slips forward you could use a crupper. On most horses the saddle slips backwards and then a good breastplate works well. I realize that you don't see this equipment at any Dressage shows, but they were designed with a purpose. In lieu of that, use a girth with elastic.
If you look up European and US Cavalry history photos, you will see both.
Certainly the Spanish Riding School uses breastplates, on occasion. http://www.luxurytraveler.com/destin...ing_school.jpg