Unlike the Federation we don't see many, if any, other species in the RSM or the Klingon Empire. 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.