That might also mean the operation is intended to be more political than military; it's not truly (or primarily) a grab for territory or a desire to take Federation land, it's mostly calculated precisely to politically undermine, or at least politically agitate, the Federation (if the choice of targets didn't show that already). Or maybe it's all to put on a show for the other nations and undermine the Federation that way. Not actually from within - undermining it from within by taking Vulcan is a means to another end, that of causing political enmity for the Federation from without. Instead of the galactic community seeing an unprovoked invasion of the Federation by Romulans, it puts the Federation in the position of having to prove its own good intentions. They respect the wishes of the member worlds - if the Romulans can demonstrate (or at least seem
to demonstrate) that there's widespread popular support for their presence on Vulcan, then they can paint any Federation counterstrike as hypocrisy and self-serving desire to hold onto territory. "See! The Federation claims to be non-imperialistic, yet as soon as we reclaim our ancestral homeland with full cooperation of local interests,
they seek to uproot us. So, Tholian and Cardassian ambassadors, I think we can agree that the Federation is showing us its true colours today, yes? Even Ambassador Spock has made an address supporting reunification, but in typical fashion the self-serving and hypocritical Federation interferes to protect its interests. So much for their supposed high moral values
Or something. In the long run, maybe Sela and co didn't care if they got a permanent grip on Vulcan or not. Either way they could have twisted things to paint the UFP in a bad light on the interstellar stage, perhaps?
The 2000 soldiers are there to make a demonstrative political conquest not a military one. I like it. It makes more sense than saying 2000 people are going to crush a planet beneath their invading boots.