Is there a way to help the ncr without killing the BOS?

as far as i've gotten so far you can do missions for both but i became a palidin in the bos and if you wear their brotherhood armor they(ncr) attack you on sight.

Yes, just do all the missions for BoS and ask the elder to join up.

according to the Guidebook, you can forge a treaty between them during For the Republic part 2. You must complete Still in the Dark(you can start and complete this before the main quest) and keep Elder McNamara in charge. He is open to the idea of forging a treaty with them against Caesar's legion.
Also complete the other quest that Elder McNamara gives after Still in the Dark to increase your fame with them and get power armor if you want.

Just something I've learned.. don't go around the BOS dressed in NCR gear.

I went to their bunker in Hidden Valley for the first time and as I walked in 2-3 of them came out with Laser Rifles and what appeared to be either a Tesla Cannon, Gauss Rifle, or something similar and was blown away within 2 seconds.

Yeah, the NCR and BoS have been at war for years.

This applies to the NCR and any NCR-aligned areas (like the Crimson Caravan) as well. I went to McCarron while wearing a Brotherhood T-51b and suddenly, everyone was shooting. From idolized to "villified" in just a couple of kills.

Just have Veronica with you. Then you can wear NCR stuff down there.

anyway to get McNamara back as Elder?

reload

I've been debating if I should get McNamara booted or not.. are there any positive/negative effects on YOU for replacing him with Hardin - or just a differen story?