Ive done many quests with NCR and have good rep with them but when i joined BoS all NCR became unconditionally kill on sight for me.

is there any way to be friendly with NCR and get the power armor training?

You can join the Bos?

Yes.

I didn't have this problem. Are you wearing armor that disguises you as Brotherhood?

i thought that was it, but i tested with no one in my party having any power armor on and they still were all hostile

Geff-Supposedly, Arcade Gannon's companion quest reassembles some ex-Enclave types, one of whom can give you power armor training. That's probably your only other option though.

If I join the Brotherhood and walk down the Strip, will the NCR open fire on me? And if so, will the Securitrons take my side? (:D)

Also, I'd imagine that when people figure out the console commands to adjust reputation, you can simply set your NCR reputation to Neutral so they don't shoot you on sight.

thanks for the replies, turns out it was just a bug, i had put on some brotherhood t-51b power armor i stole from some sleeping guy, and it said "you are now dressed as brotherhood of steel" or somethin, BUT when i took it off it didnt say the msg, so i was permanently disguised regardless of what i was wearing.

upon reloading an old state, the disguise properly comes off if i unequip the armor and the NCR is no longer hostile! ill just not put on that armor at all haha.