Can you go to a tack store and sit in some different saddles so you get a better idea of what seat and flap size you need? How tall are you and how heavy if you don't mind my asking? I know you were asking about short flap saddles, so if you're shorter you may be able to get away with something pretty small, but if not, you may find a small saddle puts you in a bad position, and the reverse if you're really short and buy something with a regular flap. What kind of budget are you working with?
I like youth saddles for small horses if you can fit and don't have the budget for a high end saddle, but I think you could get away with up to a 17" or maybe 17.5" depending on your horse's back length. I rode my 14.2 h QH in a 17" forward flap saddle, and can actually ride my 14 h pony in it, but I think she looks silly since the forward flap looks like it swallows her whole. Also keep in mind that if she's missing muscle, if you get her fitted now, the saddle may be too narrow in 6 months, so I'd either look at adjustable trees the right shape or consider going a hair wide and shimming, or possibly borrowing something for a few months. My TB came without topline and has gone up at least half a tree size since October, so it's lucky my saddles started too big.
Tug on anything at all and you'll find it connected to everything else in the universe.~John Muir