Just read the whole thread--whew! Lots of great advice, some of it contradictory, which always makes things interesting. OP, it sounds like you have made your decision and are going to give the girl a chance, but have plans in place to take action if she does cause problems.
That is the ideal scenario, IMO. Although I've never been involved in GS to me the whole point is to help girls be better people, even if they start out with some negative personality traits. In fact those are the girls who need guidance the most. Since you're starting a new year with new member(s), that might be a good time to overhaul the rules and the consequences for breaking the rules (up to and including kicking someone out of the troop), making them stricter since the girls are getting older and should be more in control of themselves and aware of others' feelings. Of course these rules and consequences would apply to everyone equally. It also seems to me like a good venue for discussing "bullying" and being "mean" and how people can stand up for themselves against that.
That is, as I said, my ideal situation. In reality, the thing that gives me pause is wondering just what kind of consequences you will be able to enforce. Not you personally, but in terms of what the larger organization's rules will even allow. Honestly, if my situation was such that I knew I couldn't discipline/remove a problem child once they were in the group, I would rather keep them out entirely. Because just letting them in isn't the part that's going to change anything. It's letting them in, and being able to teach them that their bad actions have consequences they don't like, and modeling the way they should behave instead. If you don't have the power to do that, letting her in is pointless, because she won't actually learn anything.
Since you've decided to let her in, I hope you and the co-leader can make firm plans about how the rules will be enforced, with punishments that will actually be meaningful to the girls. From the comments I've seen here I'm not entirely certain how you will be able to do that in the framework of the GS rules, but there are certainly a lot of details and nuances I don't know about, and I hope you're able to use those. I do think the girl deserves a chance, but only if you are truly able to punish any bad behavior that arises.