Gocorygogo, I see what you're saying about her avoiding acquiring more morphs than is necessary, but I don't know if this explanation necessarily stands.
First, she uses morphs for personal pleasure just as much as everyone else; Jake's the one who hounds the group the most on this point, but he gives in too. They're all pretty much equally hypocritical on this account.
Second, the wolf morph seems an odd choice for someone of Cassie's pacific values, considering it has a very nitty-gritty and personal style of attack.
I think she used the morph because it made tactical sense, not because she had an aversion to acquiring one more animal for the purpose of fighting the Yeerks. If she had wanted something less personal in the way it killed (even Rachel's elephant is less personal -- it's less gruesome to stomp someone when you can't see a darn thing than to launch your entire body at them and bite open their neck like Cassie had to do), I don't think she would've had qualms about acquiring it.
I'm not trying to say we can't read into Cassie's psyche by examining her battle morph, but there's a simple explanation of it being a good complement to the team that makes more sense to me than "she's worried about acquiring more animals" or "she's insecure around her friends."
