Methinks the Scimitar could take down any of the vessels mentioned here already, even a Borg Cube. At least, it could probably take down 2 Borg Cubes until the Collective adapts to whatever tactics it has on hand with a standard Romulan admiral in command.
Now, if Shinzon himself (or The Shinzon that was the clone of Picard if you read Captain's Blood by William Shatner) was in command of the Scimitar, I think he'd be cavalier enough (and depending on his physical and mental status - crazy enough) to give the collective a good run for its money and possibly come up with a way to defeat a particular part of the collective (he's a clone of Picard, after all.)
But, could he defeat the U.S.S. Defiant (NX-whatever it was) under the command of Sisko or even Worf? I'd say both were even money.