From what I have seen and learned, most of the pioneer Mississippi families came from Georgia, Alabama or Tennessee as land became available for settlement.My McKee family came from SC to GA into Al by 1850.
Also, most of the McKee families I have seen in the South originally came from Northern Ireland.They had came from Scotland to fight for William of Orange in the Boyne War in Northern Ireland and was given land there for their service.Mostly in County Tyrone or Atrium.Most, not all, McKee families came to America and settled in Pennsylvania and then migrated South.However, some did come into Charleston, SC and migrated from there.
Dorothy Hunt has a McKee web site and she is descended from the Mississippi McKees also. I would suggest talking to her.
I hope this helps.Email me if you need anything else.