I think Jasmine and Pocahontas would both be willing to disguise themselves and would be fine living like they would have to in the army, but Jasmine would be more willing to fight than Pocahontas, so if this included any involvement in the army, I would pick Jasmine.

There is no way Jasmine,Ariel,ect would be able to disguise themselves as a man....Pocahontas looks the Manliest and could pass, but the others would have to do some serious wrapping of thier boobs and padding of thier other parts...Mulan has a straight figure and her voice isn't too girly (like Snow White) and her facial features are plain enough (but most of the Princesses have too femine of features)...Jasmine would stick out like a sore thumb....It is one thing to be willing but it is another to be able to pull it off and Jasmine and some of the others would just call too much attention to Mulan and get her caught

I think either Pocahontas or Jasmine, but I went with Pocahontas because, although Jasmine disguised herself well enough to not be recognised, you could tell she was a girl, and I think that Pocahontas is a better secret keeper

Pocahontas is very anti-war guys, she'd probably want no part in it. Jasmine might not pass as a man, but she'd most likely help Mulan.

Ok, not taking their appearances in consideration, Belle was the closest to Mulan's situation when she decided to sacrifice herself on behalf of her father and stay in Beast's castle which is quite similar to Mulan's decision to join the army instead of her father even though she knew little to nothing about fighting. Belle would understand and be helpful.