There is also nothing caste in stone that says you have to use a bit either.
I have one TWH that wears a mechanical hackamore and the doll face I lost to a freak pasture accident, loved-loved LOVED his Dr. Cook's bitless.
He was just coming 4 and thought he'd "arrived" the day I put the Dr. Cook's on him.
My TWH that is 13 wore a 6" shank, very low port, bit when I bought him and he still does.
The TWH in my avatar is now 22 and has always worn a very low port bit with 8" shanks because he came with a Rodak motor as a coming three year old.
I suppose I could've forced him into slowing down and using a "kinder" bit on him. I can't slow myself down and I've always used long shanked bits so in our instance the 8" shanks don't begin to get on the cruel list.
When you're only 5'4" riding a horse that comes sliding in at 6:00AM from a night on the town and says "what'll we do next", having 8" shanks to hold him in check is much better than trying to fight his feisty self to slow down.
Anyway, the OP is correct to not start the filly in long shanks and she may, indeed, be happy with some form of bitless bridle.
None of my Walkers have ever had trouble "getting their gait and setting their heads", no matter what they were being ridden in