If the above quote is your evidence that "good connections to TPTB can outweigh rank and performance," then I respectfully say that that's entirely conjectural speculation on your part. The way I read that quote, all it says is that Decius may get someone to enact retribution on the commander for reducing him in rank. It doesn't say anything about said friends helping him to rise up in rank.
The ability to punish somebody for demoting you -- especially if you got demoted for doing something careless -- implies the ability to punish someone for failing (or even hesitating) to promote you. This is actually a rather common problem in some businesses, where the boss' son/daughter/nephew lands a promotion he is in no way qualified for, not necessarily because the boss is pulling for him, but because the managers of that department are afraid of incurring the boss' wrath. This is even more true in corporate cultures where the boss' wrath is relatively easy to incur and department managers are prone to just roll over and let His Incompetent Progeny get away with bloody murder.
In a militaristic and highly competitive society like the Romulan Empire, Decius' "friends" are extremely likely to lose their shit over their buddy having a spat with his CO. The same danger you face by demoting him is the inherent danger in passing him over for promotion or giving him the most important assignment all the time so he can get promoted that much faster.
there was no evidence whatsoever that Sela got her position because of her father's influence. All she said was that he was influential enough to save Tasha from death.
He saved the ENTIRE CREW from death in exchange for Tasha's ass. That pretty much makes him a bigshot. We also see a similar situation later where a Romulan general is presiding over a camp of Klingon prisoners which he has made his own personal colony; he, too, is indicated to be kind of a bigshot for the Romulans to let that slide.
Actually, when you combine that with Saavik's putative half-Romulan heritage (plus her relatively young age) and a few other tales of Romulan atrocities, one wonders if the kidnapping and breeding of alien women isn't a pretty standard practice in the Imperium. It might just be a "spoils of war" thing, in which case Sela would be a high-ranking soldier mainly because the Empire would not have allowed her to ever grow up to be anything else: she's a child of an alien captive, which means she MUST be raised from birth to be a soldier, would have been trained in a military school since she was old enough to walk, would have had her first instruction in military history in grammar school. She'd be Romulan Hitler-Youth; by the time she's 21, she'll already have a pretty extensive career behind her.
That's still more speculation, of course, but with how little we actually know about them, what else can we do?