he doesn't seem like who would choose for such a life on his own

^I can agree with you. Chien-Po is the one I am least sure about. He seems to be very kind and caring but at the same time he seems hedonistic, slothful and self-indulgent person. He is portrayed as almost saint person who is nicer and more virtuous than his friends in every way. But who knows maybe he has some flaws that Yao and Ling don't have?

I don't think that Ling had a choice. He knew that "Ping" has a living dad, didn't he? And he still caused troubles to Mulan that could easily lead to her sending from army( yes, Yao mistreated her too but Ling's tricks had much worse effect). Ling didn't have any compassion and most likely didn't relate to her. This means makes ling in the unlikeliest of them to do the same Mulan did. Sure, not all people feel compassion towards people with similiar problems but still...

I think Yao is likeliest to do the same like Mulan, Ling is the unlikeliest and I am doubting about Chien-Po.

they're all incompetent at first so I think they all didn't have a choice. Although I actually think Chien Po is the most likely. I don't think he likes to fight, but I don't think he would let someone he cares about go instead. I think he would take his fathers' place.