I've actually been thinking about this for a while now...
What in the hell does charisma has to do with keeping brahmin, all you need to do is to jerk the rope and get them moveing, therefore they ought to be strength dependant, and to a certian extent agility dependant too, and as an addition they ought to only take up one party slot instead of two, I don't really see anything wrong with that, and it isn't unbalanced because of the approximate 40hp and the inability for them to use weapons, or just remove the world limit for brahmin and set the party limmit of brahmin to 6 no matter your charisma, so whatcha think, I didn't really think it necessary to plan it out though...