So I have begun my little side quest. I enlisted Fawkes, as I happened to start it in Underworld and realize that I had good alignment before starting. Cleared that out. Started clearing Rivet City. Will need to go home and change, because I think it will be funnier if I do this wearing the Pint-Sized Slasher Mask and Blast Off Pyjamas.
In Rivet City, though, I had an unexpected surprise: Flak is essential, and unkillable. I know there will be more essentials as I do this, and the area I'm perhaps most worried about them is The Citadel.
So who are all the essentials in The Citadel, since they're likely to be heavily armed (and armored)?