What are the best perks and traits to take? What is the minimum kill count with and with out the DLCs? And I'm assuming you just have to avoid certain areas that house DCs and other uncooperative life forms!! Will difficulty effect this play through? Can I let companions kill or will I have to go alone? (I think that one depends on RP) and finally will any of the DLCs positively or negatively effect on this playthrough? I thank you in advance!

Unfortunately, this game is not made for a "pacifist" character. In theory, such a mindset in a wasteland would be considered delusional.
You are going to need to play easy, and use stealth, speech. Go high charisma/agility/intelligence. Choose perks like terrifying presence and Silent Running.

1: Good Natured is a good one to take. Black Widow, Cherchez La Femme, Silent Running, Terrifying presence are all musts. The minimum kill count without DLCs is Zero. But you must side with Mr. House. With DLCs, still theoretically zero. There are certain areas which you must avoid no matter what, and Lonesome Road may be uncompletable due to it's linear nature.

2: Difficulty affects any playthrough.

3: Depends entirely on RP.

4: Lonesome Road is particularly difficult as a pacifist, with OWB coming up second.

@Radmoon: A completely viable way of playing any Fallout except 3 & Tactics is a pacifist playthrough. It's only the entirely of Fallout 3 & the New Vegas DLCs which basically force killing. At no point in F:NV or FO or FO2 is it critical to kill someone to finish the game.

-Nukeknockout

For DLC's you'd probably have to hoard stealth boys.

You can complete the main quest without killing anyone, just put all of your skill points into noncombat skills, and choose perks that benefit noncombat skills/situations. If you find yourself in a situation where people attack you, just run away until they stop pursuing you. You won't be able to do every quest in the game without killing anyone, but completing the main quest and a few others is definitely possible. Companions will make it much easier, so that is a decision you will have to make.

As for DLCs, Dead Money would be very difficult to complete without killing anything. In fact, pretty much all of the DLCs are nearly impossible for a no kill playthrough, except for Honest Hearts. There is a weapon in Honest Hearts that can actually help you with a no kill playthrough and get you out of some combat situations without killing anyone.

Thanks for the info, but still have questions! I'm still not sure on companions yet and am I allowed to destroy robots as they're not alive?

Ignore what this guy said.
This game has been made to accomade the Pacifist. You are indeed able to complete the game without firing a weapon or raising a fist.
Sneak, Speech, Barter.

The zero kill option is possible but only if you side with Yes man and take no companion (leaving the companion kill in your name is just as killing yourself)
It is not a spoiler to say Both legion and NCR will explicitely ask you to kill someone in the main quest and block further advance if you don't.