My mare who I bought a new saddle for from aussie saddle supply who custom fit it to her from a wither tracing is getting white flaking hairs.
Sounds like a fair chance it's not fitting her. If they only went off a wither tracing, how did the 'custom fit' ensure the rest of the saddle was right for her? White hairs are usually an indication of pressure being on a certain area for a long time. As tiny said, a horse will usually show signs of soreness long before the hair changes, but horses are stoic animals & people often miss the signs or put it down to 'bad attitude' or such.
I bought a 30 dollar quilted pad for the saddle too, which I haven't washed since I got it 6 months or so ago.
Why haven't you washed it? How long have you had the saddle? Even if it was fitting her perfectly when you got it, horses & saddles both change shape over time/with the seasons, so you need to check a rigid saddle's fit every few months at least & get it adjusted/replaced if it isn't fitting. If the saddle fits well, it shouldn't need much in the way of padding under it & too thick padding can cause it to be too tight, not stable enough & can imbalance it.
One good way of checking saddle fit yourself is to use a white cloth/towel under the saddle. Tack her up with the white saddle cloth & no other padding and go for a good ride. At the end, take a good look at where the dirt and sweat marks are on it. Are they even from side to side? Are they pretty evenly distributed? Are there any pressure points or dry patches, where the horse should have sweated but didn't - sign of a lot of pressure? Of course, if you're pretty sure this saddle doesn't fit & is hurting her already, I wouldn't advise you do this & hurt her more for the sake of the experiment...
By the way I was having problems with saddles sliding forward on her down hills so I got the new saddle and that problem went away. I am just getting really frustrated with trying to fit her all the time
Yeah, one of the 'features' of stock saddles is that they have rather long forks and are close fitting around the horse's shoulders, which is great for keeping a saddle in place on a rough bush horse/ride, stockmen being able to throw them on different horses & know they'll 'grip' & stay put... not always the best for the horse though. As far as saddles slipping forward, this can be a problem with some builds of horse. Using a crupper or britching, and a breastplate for the opposite problem on steep uphills, is a good move if you're doing hilly rides.
Some horses are just harder to fit than others. I happen to have one of those too unfortunately and have tried numerous saddles on him over the years. Have you tried a treeless? They can be great and not being rigid, if they suit, you don't have to keep refitting with changing seasons. But contrary to what some beleive, I don't think they suit any horse & situation. I've tried a few different ones on my boy & unfortunately he's one of those that doesn't suit. I was starting to think the best I'd get for him would be a bareback pad! After a lot of research, I ended up making my own saddle for my boy, which I guess you could call a 'semi treeless' as it's got a 'tree' of sorts, but it's reasonably flexible.
Check out Balance International's website. They have a lot of info on saddle fit & specifics there which should help you. As with other subjects, it's important to learn the principles at least for yourself, so you can learn what's what & what is needed for your horse.