Once you've completed For the Republic Pt. 2 with a truce (instead of blowing up the bunker) then NCR and Brotherhood soldiers shouldn't fire on each other's faction armor any more. Given how buggy these things can be I'd give it a test run before doing anything important (be ready to load your last save, in other words).
You can't get the Brotherhood to work with the NCR. The whole Helios One incident?
UH yea, if the original elder stays elder he will offer to work with the NCR during the quest where you have to blow up the bunker.
Never heard of that, but i could be wrong. If that's true then possibly. Trial and error.
hardin says no. macnamara says yes