Have you not read the above posts. Im no amateur you know, i just haven't been using FRED_open for about a year now.
Beams Freed, Ordered To Attack GVD Derwent
now that you mention it, Does the IFF Unknown make the corvettes enemies to the Vasudan Destroyer?
1. If that first post was directed at me, then you should have known, being "no amateur" that you either had to beam-free the ship or select "beam-free-all" in the mission specs editor. Your questions would have been answered by reading Karajorma's FAQ, the very thing I recommended you to do...

2. Yes, AFAIK, Unknown ships are not automatically targeted by Friendly ships. They *are* classified as
"unknown".... Dark Hunter's fix should work. If it doesn't, add a goal to the corvettes and cruisers to ai-chase the GVD Derwent.