Book III was about the general legitimacy of the claims to rule. Books VII - VIII are about the best regime and the fine and noble things. In between we find the practical books are about actual regimes, and further how to restrain the brutality of politics. That is, they address themselves to the problems of civil war and tyranny. One does not give Olympic training to an overweight businessman as Jaffa remarks, but some exercise is appropriate.
Political stability comes through a moderation of claims to rule.
Extremism brings about civil war, undermines political life, and brings about tyranny. The diversity of regimes is based on the diversity of parts within a regime. Regimes vary as much as one can vary the form or arrangement of rule among the parts. This makes for a regime. There are also warriors, perhaps to be included as superiority.