Wow, there's a lot of historical revisionism in this thread.
First off, the Roman Republic was
never a democracy. It was, rather, a
mixture of democratic, aristocratic, and monarchal elements. In particular, the Senate, dominated by the patricians, was much more of an aristocratic body than a democratic one -- it was the legislative assemblies that actually provided some representation to the land-owning free men who weren't patricians, the plebeians. Those legislative assemblies didn't have power over the patricians, though.
The Roman Republic, in other words, was essentially an aristocracy more so than a democracy. Let's not try to pretend that it was some sort of shining golden age for democracy.
As for the Romulans...
Aside from some of the nomenclature, I don't really see them has having much in common with the Romans. They don't, for instance, seem to have much in the way of duplication of offices, which was a major feature of the Roman Republic. The Romulan head of government, for instance, is called the Praetor rather than the Consul, and there's only one rather than two. (Praetors in the Republic were an elective magistrate.) So they don't seem to have the obsession with balance and duplication that the Romans had. I just don't see them as Romans.
For whatever it's worth, the novels have established the existence of a Romulan hereditary monarchy, with the Emperor or Empress being the subject of an official cult, but with real power usually lying with the Praetor. The Praetor seems to preside over meetings of the Senate, the ruling body possessing both legislative and judicial powers, and the Praetor himself is canonically established to be selected by the Continuing Committee, a body which includes non-Senators like the Chairman of the Tal Shiar. To a point, there can be a struggle for power between the Emperor and Praetor, if the Emperor tries to use the great prestige of his office against the Praetor. The recent novel
Rough Beasts of Empire also established that when the Senate was re-constituted following Shinzon's assassination of them in
Star Trek: Nemesis, each of the one hundred richest and most powerful aristocratic families in the Romulan Empire appointed one of their own members to serve in the Senate. So there's no democracy, here -- it's definite aristocracy.
There
does seem to be some notion that Romulans supposedly have basic rights and liberties in the TNG episode "Face of the Enemy," but that the government often ignores those rights. On that level, I view the Romulans as being more akin to, say, pre-Regency Britons -- a monarch that views for power with a parliament, some rights but they're often ignored, etc. And, of course, the Romulans have a secret police force in the Tal Shiar that seems to be virtually above the law.