I don't think Cousins would be an issue in Boston with a stable environment full of high character guys. Plus, who would be a better mentor for Cousins than Horford?
The best mentor would be KG since you asked, but this is not a possibility now, maybe if KG retires and become our assistant coach for big men.
But even if he causes some minor problems, I could live with that, because he has all the skills to turn this team into a huge roadblock to the Cavs