I've read that the Romulans actually consider themselves the dominant species of the galaxy and that it's their destiny to eventually conquer all. In that light, comparing them to Romans doesn't really work. How they got that attitude or if it's still the "offical" attitude or what generation is debateable. The Vulcanoid species seems to have a tendency towards fanaticism, one devoted to peace and logic the other toward war and conquest. So extrapolate the Vulcan's logic to the Romulan's desire to be overlords and that's a reasonable conclusion.