Excellent point. Actually, now that I think about it, in one scene in this episode an extra randomly beams in to the transporter pad in ops in the background and walks down off the platform while a main character is speaking in the foreground. I thought it was quite a gratuitous waste of FX for just an extra in the background (though a really nice touch). So yeah, maybe there was some extra funds for different budgets that they wanted to utilize before it was repurposed for Voyager or something.
As for the extra, I hadn't seen Troopers and wasn't familiar with what McCarthy looked like without the Cardassian makeup at the time. Only pictures on the internet I saw of her were more recent, and this woman was about the right age and build. Now the question is, why did they make Jake's Season 4 wardrobe into a dress?