Unlike the Federation we don't see many, if any, other species in the RSM or the Klingon Empire.
I'm pretty sure that's just sampling effect. If I looked around my neighbourhood and generalized to all of Toronto, I'd say that the Portuguese constitute the city's largest ethnic minority. If I looked around my circle of friends, maybe a bit more than a third of Toronto's population would be queer and a surprisingly high proportion would be American students of medieval studies who converted to Anglicanism. (True story: three converted in the same ceremony.)
People from outside these empires are likely to interact with Romulans and Klingons more often than their subject species, numbers aside, because it's the Romulans and Klingons who are the people who matter most. The Kevratans and Phebens are well-known enough in their areas of space, and in a fairly meaningless way to people concerned with species suffering imperial domination (cf Tibet), but they just don't have anything like the size or the autonomy necessary to be important.
Perhaps once the Romulans conquer an inhabited planet they simply prevent them from leaving. Put a few armed stations in orbit and plasma bomb any attempt to build spacecraft. You beam down the raw materials and beam up the finished products. That way you don't get that "dirty alien" stink all through your nice, Romulan ship.
You can securly rule an insterstellar empire if you're the only ones who can get out of a gravity well.
Probably a lot of the Romulan subject species are pre-warp species, maybe most. Civilizations like the Koltaai
which are advanced enough to have a sizable industrial base and a workforce capable of extracting basic resources, but which can't pose a threat to Romulan security, might be preferred subject civilizations. Warp civilizations which can be subjugated would be more technologically compatible, but also more risky--at worse, you could have a civilization looking what the Haakona are shaping up to be during the Romulan War, an enemy that fought off its invaders and is now going on the offensive.
As for pre-industrial cultures? I suspect that worlds inhabited by these species are as likely to become colonies of mass Romulan immigration as uninhabited ones. They might be even more
likely, since they at least have established, disposable workforces.