Ok so i already had one playthrough and I was friendly with the NCR the entire game up to the end.

This time I want to go 100% with the legion from the get-go but i'm wondering how plausible this is considering so much XP comes from quests with the NCR.

On this playthrough I am just getting to Nipton ... at what point should I begin the wholesale slaughter of NCR such that i'm not gimping myself too bad...

Get a silenced weapon and start killing guys in the Mojave Outpost.

I've been against them since helping the Powders fight them off at NCRCF - very early on. I made it to Vegas with little trouble. There are plenty of quests, and plenty of ways to get experience. A few ways to cheat at it, too, if necessary.

I also sided with the Legion as early as I could in my last playthrough and I wouldn't worry. Like TES_Ronin says, even though there are many NCR quests, there's still *plenty* of things to do outside of that.

One thing though, when the Rangers come and start warning you (they'll warn you that you need to improve your relations with the NCR in a few days otherwise they'll kill ya)... Well, make sure that you're prepared for that fight because they are pretty [censored] tough at earlier levels. I found them tougher than the Legion assassins.

I killed all of them, after each visit. They tried to run, but I VATS-ed them with my melee weapon before they could escape. And then the same arrogant woman returns to warn me every time, like the NCR has a whole clone army of the same chick. When they finally dropped the hammer, I'd already killed the same squad 3 times over, so it wasn't a huge surprise.

I agree, however, that if they attack before you level up or take the right perks, you're screwed. I was saved by Super Slam in a couple of those encounters.

Yeah, if you're just prepared then I don't think it's a huge problem. I personally just kinda dismissed their threats thinking "yeah yeah...", but of course they decided to roll around just as I was sorta in bad shape from just finishing some other area (can't recall which one). I had stims but no Doctor's bag or Hydra left and running low on ammo for my more suitable weapons.
Was a great feeling to survive it though and limp back to town half-dead, hehe.