Now, Lets look at the Romulans and Klingons!

Romulans:
Executive:
Praetor, Pro Counsel, Cabinet
Legislative:
Senate, with senators usually from powerful families, maybe some genuinely elected ones.
Judicial:
Probably.. Lawyers are like space roaches.. there everywhere!
Now, the Empire, it is a semi democracy, but more of a "One Party" Chinese model democracy.. Tow the party line, or get exiled.
As for colonies, or subjugated planets, you have governors, like the old British Empire model, of governors being sent by the empire to rule over the planet, no real "Self Rule". , Central control and power.
Still have money, jobs etc, like the Chinese/soviet model, you have a few families/people that control most of the government. Like ww2 germany, you have the SS or Stasi in the Tal Shiar, making sure the party line gets towed by the masses.
Klingons:
Executive:
Chancellor of the High Counsel, more than likely some type of "Cabinet"
Legislative:
High Counsel, members of the great houses, maybe some type of "Elected" seats.. maybe
Judicial:
Yep.. more Lawyers!
Now, the Empire... More of a fuedal system, have lords and kings, controling the land, money etc. there is a central power that sets the rules, you have governors of planets, if its a subguated planet, its a governor that has been sent in, if its a colony, you might have an elected governor, or one that is "Placed" by whichever "Great House" is in power on that planet.
Military, you have a central military, but you also have ships that are part of the great houses, that I gues "Loan" them to the military. No real distiction between the Military brass and the high counsel.
You still have money, but more of a Feudal system, in that the Great Houses control the land, etc. so a low house may have to align with a great house, have its sons serve there military, etc to get a parcel of land, some money etc.