Yes, more dominant horses may help in your filly's case, since she has learnt from these more passive horses that she can indeed be a herd leader; BUT on that token, because she is young, she really doesn't even understand the true dynamics of a herd, especially if she hasn't been in with other horses. So a more dominant leader who will stand up for him, or herself could be the key to putting your filly into a better understanding of what herd dynamic really is. But again, this is nothing you can actually do, other than finding the right sort of horses (ones that won't necessarily hurt her, but will not allow her to bully them either, and help her to understand the herd dynamic better.
My mare is an extremely passive type; I keep her penned separately because if I don't, she doesn't get enough to eat, and gets bitten up pretty good; even when she is in with other passive horses! They learn quickly just how passive she is, and will take advantage of it. She won't fight anyone at all...might squeal when someone is introduced, but she doesn't fight for supremacy.