Everyone starts somewhere, and of course they can grow in their career. I just didn't see any indication that T'Rul, sent to babysit equipment for the enemy, would return to coordinate military activity at an international level. Besides, just because she's in possession of a relatively high rank doesn't mean that she came by it the way a Starfleet officer would. The Romulans may have a very different military culture than Starfleet. Consider that Sela was very high positioned at the tender age of 23. Did she earn it by climbing the ranks, or was it achieved by political and family connections? How much merit does it take to climb the Romulan military hierarchy compared to simply knowing the right people?