Re the Romulans: if I recall correctly, Admiral Hansen said that they were considering opening talks with the Romulans during BOBW, which is sorta indirect precedence for their appearance here.
As for the runabout
: I agree it's a plothole. You can try to rectify it by arguing that it shows the Dominion's arrogance, but I'm sure not I buy that. (And I doubt it was the writers' intention.)
On the other hand, though, you could argue that the runabout is one of various hints dropped throughout the series that the Dominion is decaying.
Off the top of my head:
-they didn't know about the wormhole, and weren't expecting the Federation to show up for centuries (nor were they expecting Odo to show up)
-multiple instances of rogue Jem'Hadar
-the fact that they didn't think to strip the asteroid's facilities of its electronics, thereby allowing prisoners to send out messages not once, but twice.
-the total mission failure of the crashing Jem'Hadar ship in "The Ship"; seriously, a vessel carrying a changeling suffers mechanical failure and crashes
? Far from Dominion space, no less.
-Eris's lock. You couldn't have put something fancy-looking inside that takes Quark longer to figure out than the time it took to get back to the station?
I doubt it was intentional, but if you choose to read into it, it begins to look like the Dominion is not what it once was, despite its illusions to the contrary.