You can always try. My observation is that wintecs have big poofy panels that can look large on a smaller horse. Also the panels can’t be opened up and restuffed like a leather saddle. I still think you will do better with a quality secondhand saddle.
I also see that this horse has poor topline and a prominent backbone. Do you think she will develop muscle over time or is this just her? You might consider half pads with shims to fill in those hollows to help with saddle fit.
Edited to add that I just noticed you said back sore. I would really try to get on top of that with body work, lateral gymnastics on the ground and checking her nutrition levels including magnesium and protein. I would also not ride until her back issues are resolved. If they don’t resolve with massage and no riding, then you need to look at hoof angles and if that doesn’t fix it, start on rads and diagnostics with vet. Also let the horse get plenty of turnout and rolling.
What I’m seeing as poor topline could be the back clamped down agsinst pain.