Damn shame because with due diligence people could trace this fortune and "if"/how many slaves this Mellon family actually owned at one time.
They immigrated into Pennsylvania long after the Quakers had banned slavery and they were mostly into banking, so it's unlikely they owned any slaves directly. However, it's possible that they may have bankrolled slaveowner enterprises (I don't really know how involved Pennsylvania banks would have been in Southern enterprises in the 1800s).
However, it's definitely true that Thomas Mellon had a hand in promoting segregation in the post-Civil War era.