For those who are curious, it hasn't gone so well.
I installed the RP-start mod, but when I select "Fiend" the game crashes faster than Lindsay Lohan's ferrari. Can't be a conflict because this is my first mod for this game. The creator of the mod is aware of the Fiend problem but I won't hold my breath for a fix as it looks to be a complicated one. Many others on the TSnexus forum with the same problem.
So I started my character in vanilla and looked for an end-around. No way, so far as I can tell. Fiends attack regardless of what I'm wearing (Raider armor, Khan armor, etc), although from inside Vault 3 they will leave you alone. I thought maybe after selling some drugs to the Vault 3 boss they might accept me, but nope. There are no Khan quests I can see that will get the Fiends on your side.
So the bad news is, until a new mod is made or the existing mod is fixed, no way to hang out around Fiends without getting disintegrated or whacked with pool cues. The good news is I like this character anyway, so I'm playing away. This is my first real EW character, and these things are not as overpowered as I thought they were, at least in the early going. The NCR attacks me on sight, as does the CC. I'm addicted to whiskey and psycho. It's 180 degrees different from my prior characters. Going to run HH for the first time with this guy, I think.
Since you're on a PC, you can do the following in the GECK:
1) Set up an .esp file, call it FiendStart or something like that. Save it and close the GECK.
2) Reopen the GECK with the new .esp file set as Active.
3) Click Player on the taskbar, then click Faction. Right-click and select 'new'
4) Scroll down to FiendAnimal Faction and drag it into the Player faction window. Do the same for FiendsFaction NV. Set them to 'Neutral' or better.
5) Save the file and close the GECK.
6) Check the box for the new .esp under Data Files in the start menu.
That should make the Fiends at least Neutral to you, which means they will not shoot on sight anymore. This will take maybe 5-10 minutes to do and will make it possible to RP as a Fiend without getting blasted out of the RP the first time you encounter some. Note that their dialogues, for those that actually have any, are not set up to recognize the player as one of them, so don't let the fact they will not actually accept you as one of their own affect the RP.
Depending on how strictly you want to RP this,you might want to limit yourself to Raider armors and tier 3 weapons (Energy or Guns, doesn't matter); this will not be practical to follow during
Dead Money and
Honest Hearts, so plan accordingly.