I actually had considered that for a while, Dark. I've since learned more about it, so I don't know how accurate it would be, but my idea in general was that society would be divided into several strata - The grand 'king'(which would of course simply be a guardian, with the alpha mare being the real leader) would send his sons out to become kings of smaller cities, who would send their sons to be kings of smaller cities, etc. etc. etc. So you would have several 'rings' for the males, who occupy roughly the same social status with others within their rings. Upon their fathers death or promotion, sons would vie to take up his position(Which could, inevitably, lead to a
Klingon Promotion situation), making their way up through the stratas. Mares would have internal hierarchies within the cities, a system that would more closely resemble our society, except that when a female reached maturity she would need to go and find a new city, or to meet up with a stray male and several other females, and they would eventually start their own city or 'herd'.
Males who didn't take the role of protectors of a city would form bachelor packs, becoming something akin to nomadic barbarians. This would be seen as a cultural training period for them to gain the power to become protectors in their own right. A lone stallion could challenge the leader of a city for the right to be guardian, and if he lost he would leave, or be killed, or other things(Depending on the temperament of the current guardian) and if he won he would take over and, as with the reverse, the fate of the old guardian would depend on the whims of the new guardian. If a guardian made it to old age and it became clear he would soon be too frail, he might opt to take on one of his sons who had not yet taken a city or been cast out on as an apprentice, and teach him to be the new guardian. Alternatively, he might choose a son who has a city to mark as his successor, and that son would be moved up to his old city whenever he decided to retire.
Basically, the stallion system is the most complicated aspect. To put it in a more organized fashion, the Stallions send their sons out, and all the sons form nomadic packs. A stallion who feels strong enough will break off of the pack and challenge the leader of a city. If he wins, he becomes the guardian of that city. When his father starts to grow old that son is now in competition with all the other sons who are not currently in nomadic packs for the father's position. If he takes over, his eye is then on the throne of his grandfather, or great uncle, or what have you. Guardian stallions can challenge their direct superior(Father, grandfather, great grandfather) for the right to their throne, but they cannot challenge for the throne of an unrelated stallion, unless they're nomads. Nomads will generally only challenge for the lower thrones, but it's not unacceptable to challenge for a high throne from nomadicy(Though inadvisable, since the higher thrones tend to be strong and experienced).
The matter of how those sons get made is... more complicated. One idea is that the guardian has Harems, or can choose from the mares in the city. Another is that he breeds exclusively with the alpha mare, but that's hard to say. It's alos possible that stray mares could breed with nomadic stallions, and take their children into a city. Those children would be common born, and have no real chance for advancement until they became nomads themselves.