I plan to get into a horse trade school (Meredith Manor) when I’m older where I would ride multiple different horses in one saddle meant to fit to me (in English~jumping and dressage). I’ve been told to buy different fitting pads to adjust the fit of the saddle on the horse, but I’m not totally aware of how that works.
Pretty much, what I’m asking is how I would fit my 17.5" jumping and dressage saddles to different sized horses?[/QUOTE]
One saddle will not fit all horses. Your best bet is to buy one with a medium/wide tree and invest in a saddle pad that has shims. If the saddle is too wide, you can use shims to make it wider, keeping in mind that it is important that the saddle be balanced and that the panel configuration and tree shape works for that horse.
A saddle with an adjustable tree could help but most of them are not meant to be swapped out daily for different rides. Also, as mentioned above, changing the tree width alone won’t guarantee fit.
You cannot make a saddle that’s too narrow fit a horse comfortably.
See if you can spend a few hours watching someone fit and adjust saddles. You need to really understand what works and what doesn’t work so that you don’t exacerbate a problem by creating different pressure points.