Unfortunately, I think a 17" saddle would be too big for this pony for two reasons:
1. it would extend too far down her back and put pressure on her loins. A big no-no as there are no ribs here to support the saddle and it can cause brusing. We have a 11.2 hand pony and my 17" saddle is too big for him.
2. big saddles can have a tendency to slip forward on small ponies. But it really depends on the individual pony and saddle. This happened to me once- we were trying out a saddle on my first pony about six years ago and as I was trotting along, he suddenly put down his head to graze. The saddle slipped forward onto his wither and I somersaulted over his head! lol I wasn't hurt and guess it would have looked pretty funny!
But yeah, it really depends on the individual pony. But I'd say probably not.
When I bestride him, I soar, I am a hawk: he trots the air; the earth sings when he touches it; the basest horn of his hoof is more musical than the pipe of Hermes. ~William Shakespeare