yeah, I'd be against reducing the HP on these; for now, I'll be avoiding that.
Honestly, I think a flag on these isnt really necessary. I just think the rooms in question should be edited a bit. So there's things keeping the Fearsomes from coming together. When you have to fight them one at a time... even if there's small foes around... that's fine. They're designed to be fought that way. It's when two or more of those huge guys get together that the imbalances appear and go out of control.
Also: Those black hole things. Those + big foes need to be a major no-no. There's already some big balance issues with those in some rooms, but... when any Fearsomes or Chaos-adders are around, things just go bad fast. And I mean bad as in "broken". Those need to be around small enemies only.