@Kor - That has always been what I assumed
Remans were; I'm glad in the Pocket Books continuity at least, it has been established - they also suggested that those
Romulans who possess a forehead ridge may have acquired that trait from another Vulcanoid people - the
Debrune - the same civilization that left ruins in the Neutral Zone in "Gambit, Part I and II". In my head canon, I also include another Vulcanoid offshoot people in the Romulan Empire - the
Garidians, from Star Trek: A Final Unity. But it is also possible the Romulans and Remans have some small alien admixture; their proximity to Vulcans means I don't think the Romulans are very genetically distant however - they lack telepathy and ponn farr, but otherwise appear physically almost identical to Vulcans.
As a kid, before Nemesis came out, the consensus I think most of us had was that the Romulus system was unusually blessed with two M-class planets, right next door to each other, and that the Romulans thus had an advantage over most other races in their early history, being spread across two planets.
I can't say I was really happy that this concept was dropped in favor of a Reman sub-species, at least at the time, because I thought it was a little off putting that the Romulans had an entire planetary slave class in their own system; again it was one of these times when Star Trek lapsed into playing the space fantasy game, by bringing back archaic social systems in supposedly advanced times. But I guess it is not impossible.
It could be that Romulans are a majority on Remus too, but that the Remans are considered the native ethnic group, having been descended from Romulan laborers in their early history. It could be that they are one of numerous auxiliaries in the Romulan Empire. It could be that Remus is the equivalent of the pre-Civil War south, with rich Romulan land owners, who have a lot of influence in the senate, owning plantations and living in large manors, employing the Remans on their dilithium mines.
In classical societies you had some very stark relationships between different classes - such as the famous case of the Spartans and Helots - but I can't say that I liked the idea of Romulans being brought closer to classical inspirations, just because they had the Romulus/Remus as their namesake. But now that it is canon, we have to accept it and move on.
I think Nemesis should have been more about the Romulans themselves, not a subject species, and should have been epic in scope, with perhaps Commander Sela returning as the final TNG villain, wrapping up a long thread, and giving Picard and crew a Undiscovered Country style sendoff/closure. I'm sure a lot of people feel the same way - a disappointing missed opportunity to seal the Next Generation era.