They're the bare minimum level to be prostituted.
Well you need to up their level.
I haven't looked at the the code but I think their training skill level and their slave level should effect the RNG.
Oh and they will walk up to you to give you the credits. I'm still not sure completely how long they're gone for but I think they work for 24 hours.
The issue is when they're trying to give me the credits, I don't receive them and I'm sure how to, I've tried talking to them but it doesn't work.
They are the ones that have to initiate the conversation for them to give you the credits.
