As the title says. I even used the Great Khan simple and warrior outfit, the Powder Gangers guard armor and Legion armor, along with letting go Boone and ED-E. And yet, the Khans are still hostile with no option for negotiation. a Any suggestions?

Also I'm playing the 360.

why are they hostile ?

you have never met them before ...or
have you been a naughty boy

What is your reputation with the Khans, before you even start on this quest?

...

Never encountered them before I went to the Tribal Village up north Goodsprings to get armor out of desperation and there were only corpses, NCR armor is not in my inventory, and I even went to Novac and Primm to terminate Boone and ED-D's contracts and watch them go back to their rooms. I honestly have no clue why they're hostile even when I pick Lt. Monroe's negotiate option.

you didnt steal anything of thiers did you ?

thievery is a quick way to annoy them

I don't think so? Unless the Khans are camped south of New Vegas, I've only been exploring the lower half until this point. :T