I think that was because the Obsidian Order weren't allowed (even by Central Command) to have their own ships, and so they had to use their own operatives as the crew for all the vessels. The reason the Obsidian Order was defunct after the Founder Homeworld incident was because nearly everyone in the Order was now dead, not just a scandal. The Tal Shiar may not have had such restrictions and maybe even authorization from the Empire to attack (The Romulans did try to blow up the wormhole before and that the normal military).
The Tal Shiar must've been its own army and navy from its founding, or the ships and their crew were "on loan" from the actual military, while the Obsidian Order put most of its eggs in one basket by secretly and illegally assembling an armada that got decisively annihilated.
Either way, the Romulans often came out of direct fights with the Dominion much better than the Cardassians and Klingons, that's for sure, and their fleet seems relatively unscathed (and expanding).
Not long after the Dominion War in Nemesis
their shipyards turned out a new generation of Warbird in significant numbers, the Valdpre
-class, and hulking monstrosities such as the Scimitar
-class and the "mining vessel" Narada
, which both seemed one of a kind and were comandeered by renegades. The Valdpre
-class and the wankish Scimitar
were most likely the result of the Romulans reverse engineering Dominion ships (which were in some ways more advanced and could be built on a larger scale than most Alpha/Beta Quadrant warships). The Narada
was once a normal industrial ore processing vessel and was like a smaller brown Warbird, but captured Borg technology was intergrated into it and it grew much bigger.