I think that based on the vet's reaction to humans and Calsha's relationship with her slave that it's clear that these people are the REAL slavers who can enslave without chains. But at the same time, Sky says that the others don't know how good they have it because only Dareh has owned them, and the slave that attacked Xander seemed pretty scared. So I get the feeling that all these slavers have a different philosophy, but there's still a wide range of treatment. And yes, I see Dareh as the strongest and most dangerous of the slavers. If Xander had spend six months in Dareh's stable instead of Leshar's, he would have been beyond saving. He would have chosen to stay in the safety of that stall, just like the women would.