I am considering a story where a group of human colonists looking to colonize an alien world come across another human society on an alien world on the other side of a wormhole. The colonists crash land on this world after an incident and discover this society. As it turns out, the humans on this world are actually descendants of many people abducted from earth by alien "overlords" over the course of several millennia seeking to use them for a social experiment.
The purpose of said experiential was to see how these people, from various regions of earth and time, would interact with each other over the course of these millennia. Fast forward in time and these humans and their descendants have populated this world, known as Rhye, and have become rather technologically advanced, say beginning of twentieth century Earth, but with some near future technology. Keep in mind this tech is at least one hundred years behind the tech the colonists have. The colonists, however, despite their shock to see this human society on this world, discover that these people basically recreated Earth. There are nations, borders, wars and even a UN often referred to as the "Planetary Council".
But what really catches their attention is the fact that these humans, given the different eras of human history they came from, have developed a medieval oligarchy type society where social class and upbringing determines your status while capitalism and democratic/representative governments still exist.
In this system, social rank is predetermined by birth. You have a role and you are to fulfill that role regardless. There are corporations and capitalism to certain extent does exist but they are tightly controlled by the Elites and influence the government. Industry and networks are controlled by them and the only way one can hope to advance is to get a better standing within a company or business. Think of the social situation in the movie Elysium.
The governments are typically run by the more wealthy and superior classes comprised of the following branches in ascending order of power: the People's Congress/Parliament, the Military (which I will go into detail about later), the Hierarchy (which deal with judiciary matters, law enforcement and government management), and finally the Head of State who is in charge of running the state. The upper class protect and help provide for the lower class in exchange for labor. These same Elites make up the Hierarchy holding various elected government offices. These Offices include: Council to the Head of State, ambassadors, attorney generals and high ranking generals, law enforcement, Homeland Security, etc. However the people do not elect the Head of State but rather the Hierarchy. Now, the Congress or Parliament, is elected by the lower class and typically is comprised of lower class politicians. Elites tend to stay away from it considering it beneath them to be a part of. However, while these congresses and Parliaments have the power to draw up legislative proposals, they lack the power to enforce them or pass them. Not even the Head of State. Only the Hierarchy does. In essence, they only exist to inform the Hierarchy of the people's wishes, wants and needs as a way to keep the population content.
The military situation is fairly unique as well. There are federalized militaries made up by the privileged upper class aka Elites that protect the states/kingdoms/republics. However, while these militaries are loyal to the states, they are not commanded by the Head of State but rather by the elected officials in the Hierarchy who either fund or otherwise influence them. Ideally, the Hierarchy answers to the Head of State but one can imagine how some corruption can come to play here. However, there are also private military organizations or "orders" that offer protective services to some republics and kingdoms in exchange for funding. And given this is a society originally built by people from the more ancient to medieval eras of human history and this world itself is advancing out of its own medieval era, the organizations tend to follow a Templar-style set up and often function like one. So one can image how complex this system is.
Now, as good on paper it sounds to me, I still am not sure how practical this is. So, is it entirely feasible for a relatively stable society to exist with this kind of "Medieval Post-Feudalism Democratic Oligarchy" system? Please let me know if I need to add anything detail wise if you need clarification.
Edit: I understand oligarchy and democracy are polar opposites in a way and that there is a bit of confusion here. But just work with me. I want to know if it is a relatively stable system that could work for some time.