Most characters of roughly the same alignment will get along, problems usually arise between characters that are good and characters that are evil. If both are good or both are evil, you're usually fine.
Also I think that the new NPCs do not in fact fight with anyone regardless.
Maybe I was unclear. A vampire women talked to on the graveyard and said join me and leave the shadow thieves. Since I got Edwind from the shadows thieves I thought maybe he would leave if I joined the vampire women.
Edwin works for Edwin, truth is he sticks with you because you are powerful and if his old cronies, the Red Wizards of Thay, turn up he needs some powerful allies to help him. He doesn't care if you side with Bodhi instead of the Shadow Thieves, he was just using Mae'Var anyway.
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Most characters of roughly the same alignment will get along, problems usually arise between characters that are good and characters that are evil. If both are good or both are evil, you're usually fine.
Also I think that the new NPCs do not in fact fight with anyone regardless.