I assume it's a rather well known bug but I'd like suggestions on how to fix it (PC).
I really, really don't want to fight them.

EDIT: if it helps, I noticed that only Moreno and Johnson are hostile first, then the rest when I return fire.

could it have something to do with the questline you've chosen? i haven't gotten arcade as a companion yet, but afaik the remnants have a ncr allegiance, so they are maybe only friendly to you, if you choose the eureka quest.
just a guessing, though

I followed the quest line for mr.house and i got help from the remnants. Just before crossing the bridge thing to the last area where you fight ( forgot the name). The remnants started shooting at me and the robots shot them, after that hell broke loose.. i found my self fighting legion and remnants with gatling laser.
I felt happy actually when they attacked me Mostly because it ment i could kill them and take their remnants armor and the gatling laser to this ( forgot a name again). And the ramnants and mk.2 robots cant follow you to the last area.

Cannibal Johnson will go into the Legate's Camp with you......Doc Henry stays outside the Camp and Moreno and the LT stay on the Dam.

aaah i didnt see cannibal johnson follow me to the legates camp.. i think it was because i killed him, mmhhmmm that might be the reason. Well anyways i kicked every ass inside the camp soo

I had Cannibal Johnson and a MK II with me when I confronted the Legate. As my speech skill was high enough I didn't have to fight him. A shame really. I wanted the mask.

Why didnt you just blow up his head ? with cannibal johnson and a mk II robot he would not stand a chance against you...

( for those who needs a quick way to kill the legate: get a gauss rifle or the ICS, i prefer the ICS and sneak to a good point where you can snipe him with a critical hit to the head. Takes aproximently half of his life )

meh, sneaking. there is always the chance to easily turn him into an ash pile with the alien blaster, that never gets old

The reason the Remnants are hostile is because you made the NCR hostile and the Remnants are considered NCR faction. The only way to keep them friendly is to use speech on the NCR Power Armor troops near the console and get them to leave peacefully to keep the NCR and Remnants friendly. See how awesome Speech is

I wanted to see what speech would do. I'll have more opportunities to kill him in other games, and not have a speech that high.