I can't help but think you are going to have saddle fit issues......not from the pad but because of her conformation and lack of weight. At the very least I would pad her up with another thin pad or nice quality saddle blanket. If that doesn't work well, maybe invest in a pad specifically for sway backed horses.
She is really going to be a candidate for saddle bridging because of her sway. I have a Mustang who isn't nearly as swayed and a saddle shop told me any saddle I put on him is going to bridge because of his back conformation. I did prove them wrong and I did find a saddle that fits great....no white hairs and a good sweat pattern but I probably went through (no exaggeration) about 10 western saddles.
Your girl has two things going on........a sway and lack of muscle/weight. So at the very least I would use a high quality pad (or two if they are thin). Wool felt is the best. And then if you have problems long-term, try a sway-back pad. They do make them for western saddles but you have to look.
If I'm not mistaken, the pad you are using is a Professional's Choice air-ride? I have a couple myself and consider them pretty thin. They are fine if your saddle fits your horse perfect, but there is really no margin for error. I usually put a wool saddle blanket underneath and the air ride on top.
Good luck with your pretty girl.