You can't have a "star empire" that's only on one planet.
Then again, there were the Vorlons from B5 - they were called the Vorlon Empire, but for what reason? How imperial were they? The Centauri acted more like an empire than the Vorlons did, and the Centauri were a 'Republic'.
1. Were the Vorlons actually limited to only their homeworld?
2. The Vorlons could get away with calling themselves an empire because their technological base was so vastly superior to all of the other species, even the Minbari. If you can annihilate planets at will--the B5 sourcebook suggests that the Vorlon planet-killer could destroy planets like
Jupiter--and have an otherwise vastly superior technology base, you can call yourself whatever you want.
^Heck, it's right there in the name. You can't have a "star empire" that's only on one planet.
And you can't have Slaves of the Carnelian Throne without slaves. Of course, being an alien race, their definition of slave is different than ours. In the same way we're using an human word, Empire, to define an alien government. Sadly, our TVs aren't equipped with universal translators so Rimulan is rendered using imprecise human terms.
Good point. But ...
There's no question that the Romulans have colonies. The question is how many compared to the other governments and what sort are they? If their region of space is primarily metal poor stars (metals in astronomy terms being any element other than hydrogen or helium) then most of their star systems may have nothing but gas giants and small moons buried in intense radiation belts. Habitable planets may be considered too rare to waste on the lower classes.
1. According to the
Star Charts that's been used to guide Trek literature writers, the Romulan Star Empire occupies a vast volume of space. It's very unlikely that such a vast volume of space would consist mostly of metal-poor stars, especially since the named stars inside the RSE do seem--on a quick glance, admittedly--to be about as metal-poor or metal-rich as stars outside the RSE.
2. There doesn't seem to be such a tight relationship between stellar metallicity and the likelihood of planets as you suggest--see this
post and discussion[/URL] at Centauri Dreams[/URL]. Metal-poor stars are less likely to have planets than metal-rich ones, but even the poorest can have small rocky worlds.
3. Reasons for the Romulans--by any standard successful and reasonably pragmatic imperialists--to limit the spread of their population away from their homeworld to other suitable planets are lacking. Class prejudices are unconvincing: there are no real-world precedents for Earth imperial powers doing that (if anything, substantial evidence to the contrary) and no evidence of other Star Trek imperial powers doing that and no evidence that the Romulans themselves would do that. Besides, even if the lower classes were prevented from leaving the Romulan homeworld, what would prevent the upper classes from migrating to new worlds?
4. There's no incompatibility between the Romulans ruling over multiple subject species and the Romulans having large populations offworld. If anything, having large subject populations, whether Remans or non-Vulcanoid conquered species like the Kevratans, with already established economies and built-in niches for ruling-class migrants, can attract large populations. Two decades after apartheid's end, for instance, there are still substantially more whites in South Africa than there are people in New Zealand or Uruguay, two other Southern Hemisphere destinations of choice for immigrants from Europe.
5. In any case, Trek literature has identified multiple Romulan-ruled worlds as having substantial Romulan populations. The proportion of conquered planets with a Romulan settler minorities (or majorities?) to originally uninhabited planets that were colonized by Romulan settlers is mainly of sociological interest. (I'd expect the number of pure colony worlds to be higher, mainly because of all of the non-Romulan species that would have been preempted in their expansions.)