If all they required was someone who believed and she smoked already like mama Cecile?
shareWas Hallie the black nurse that told Caroline to get out of the house? I assume because she knew evil Hoodoo was going on in the house and made the smart decision to run for it. Violet and Luke needed someone ignorant of the local customs and that's where Caroline fit the bill. She was naive enough.
Yeah the black girl's mom told her bad stuff happened in that house. So she pretty much knew what was going on and wasn't going to stick around to find out.
shareHailee is the girl that quit. She appeared to be sort of local, enough for her mother to know rumours about the house. So when Hailee realised what was going on, she quit before they could use her. Justify says at the end that the black ones always end up leaving - because they've been raised with knowledge of Hoodoo and recognise the danger signs.