I did a little reading on the matter when I first saw the thread, and it appears to me that, in the Roman Republic, a praetor was an appointed official who acted in the consuls' stead when they were out of Rome, with the army. I also found that the praetors were also sometimes the commanders of the "armies of the state." Later praetors were appointed to handle judicial matters between "foreign freemen" (peregrini), or between peregrini and Roman citizens. In the imperial era, it appears that praetors were appointed to administer justice in each of Rome's territories. Some emperors had as many at 16 serving under them.
That's the historical. I haven't really spent time studying the Romulan model (i.e., it's been a while since I cracked open my Icon "Way of D'era" or my FASA "The Romulans"), but I was long under the impression that the "canon" Emperor and Praetor were one in the same.
StarTrek.com mentions the "Proconsul" in Unification, Part II, and the "Praetor" in Nemesis. Is the former the Senate-elected governor of Romulus, or the Star Empire? Is the Praetor the imperial dictator, or the appointed "commander of the armies of state" and national judicial magistrate? Do they both exist, in similar fashion to the two consuls of the Roman Republic? Or is an imperial Praetor superior to the Proconsul, like a head of state and head of government (monarch and prime minister)?
Given that TPTB aren't following a strict Roman model, I'd say that the latter is more likely true. A mostly-hereditary (subject to the approval of the Senate) imperial monarch (Praetor) is the head of state, with functions and powers commesurate with such a title. The Proconsul is likely the head of government, leading the Senate and executive branch.
So, in that vein, "Emperor" and "Praetor" are just two names for the Romulan imperial monarch, and the "Empress" is simply the Praetor's wife. There's probably a give-and-take between the Praetor and the Senate, as a strong, charismatic monarch may have several allies in the Senate, while a weak monarch may be a puppet of a small group of influential Senators, who wish to exert control over both branches of government.
Just my thoughts.
Davy Jones
"Frightened? My dear, you are looking at a man who has laughed in the face of death, sneered at doom, and chuckled at catastrophe! I was petrified."
-- The Wizard of Oz