Romulans separated from Vulcans when the Vulcans adopted logic. Constrained and regular hair seems at odds with a people described as "passionate" and "violent". Does Star Trek ever (canon or no) justify their generally conformist grooming? I'd long thought that Romulans should have manes of hair rather than the closed cropped style that they maintain. Something along the lines of the female Romulan commander seems more in-line with their supposed character. Even more variety in short hair would seem...dare I say...logical?
I can only remember meeting military Romulans (in the broadest sense) watching Star Trek. While the attributes you describe are supposed to distinguish them in character from Vulcans, they are still a disciplined military state (assuming the narrator is impartial and doesn't convey Federation propaganda). The trimmed style of their hair reflects this.
Out-of-universe, Vulcans and Romulans are physically remarkably close to Humans. The only real physical distinctiveness are the ears and the eyebrows (inner physiology and blood colour are not visible normally). So, in addition to these weak indicators I assume they added the hair line to reinforce their relationship for the audience.
Like bitmask has already speculated, their short trimmed and well groomed hair in males and females alike is more likely to be influenced by the militaristic society of the Romulan Empire.
As to the claims about them being "violent" and "passionate", one would have to consider what those two claims mean and who claimed them.
Romulans lacked the rigorous mental disciplines developed by the followers of Surak. They were a passionate people, easily moved to extreme emotions. (TOS: "The Enterprise Incident") Link
I can't quite remember who said this but it was either Spock or Kirk and there are issues with how much stock can be placed in either of them stating this in a VERY strange episode with a lot of odd out of character behavior amongst the main characters.
Spock being part Vulcan is immediately biased to view Romulans as passionate and violent in comparison to Vulcan's. He likely could say the same thing about Humans.
As far as Kirk, he acted very out of character for that episode being irate and acting reprehensibly by single-handedly invading the neutral zone to try and steal a cloaking device. The episode begins with Dr. McCoy being concerned for his mental state and well being.
I'm concerned about Captain Kirk. He shows indications of increasing tension and emotional stress. I can find no reason for the captain's behavior, except possibly that we've been on patrol too long without relief and diversion. He has resisted all of my attempts to run a psychological profile on him.
His actions in this episode very nearly bring the Federation and the Romulan empire to full scale war yet again. Any remarks he may have made about the violence and irrational passion of the Romulan people come off as a tad hypocritical from him in this episode.
In short, I would say that the Romulan people are indeed violent and passionate like many humanoid races, and certainly more than the Vulcans but not quite as much as the Klingons.