Mul clarifications:
Origins and ownership:
You can only be raised in the slave pens of Borsail, Kasix or Winrothol - they're the only ones with the know-how to breed, raise and ween muls.
Thereafter, you MAY be sold to other noble houses, great merchant houses, the T'zai Byn or even certain ministries in the City. However you would not be sold to some merchant or rich individual, for "private use".
Bond-mate and relationships:
A mul would always have a bond-mate. That is legit the main tool with which they can be raised, trained, disciplined and made to suffer. That said, muls could be raised in the company of others, to provide an added leverage of control to the owners. But these "others" even combined, would not be enough to replace the bond-mate.