I am somewhat unclear about the legal status of slavery in the NCR. Is it completely illegal? Regulated? Permitted? You don't see slaves in the NCR towns of New Vegas, but Slavers' Guild operated in the NCR territory in earlier games, and it is quite clear that prosttutes are sold to Gomorrah rather than hired...

Probably completely illegal, given the NCR troopers hatred of the "Legion Slaver Scum". In earlier games NCR was a LOT smaller but they did fight the Slavers' Guild but they did so by using the Rangers. I'm also certain that the NCR government isn't in the process of selling six slaves to Gomorrah, their most likely sold by Fiends or mercenaries. New Vegas isn't even part of NCR so how would they regulate that anyway? They probably wouldn't and couldn't, given the terms in the Treaty of New Vegas and the troops very apparent liking of the place.