I suspect my stock saddle doesn't fit Lisa well because it slips from side to side when my son or husband ride. However, I am not going to spend $ on a different saddle for the few times (like 5 times in a summer) that those two beginner riders go riding since the saddle doesn't seem to hurt Lisa at all. This is the only saddle that they like also. The typical western saddle bothers them in the crotch.
Lisa doesn't have a lot of withers, but she does have enough. I have no problem with my western saddle on her. She is pretty wide and round in her back compared to my other skinny-mini horse for sure, but doesn't seem excessively so compared to other people's horses in general.
So, in an effort to stabilize the stock saddle, I bought a neoprene girth for it. That didn't help much. I am going to use a thin wool pad next time and see how that goes. I am also considering buying/making the full rear harness that can be had for Aussie saddles. Do you think that the belly strap and butt harness will help?