Ancestory of Daniel Mackenzie Harris

We decided to start with my son so that the research could include maternal as well as paternal sides of the tree. I dont know right off if thats normal, but it seems like it to me.
My grandfather William Orgain Harris II had begun tracing roots back to Robert the Bruce and beyone, but there are spaces that I cannot link at the moment. (Task1)

The Bruce's are liked via maternal side of the family and Grandfather has only traced the actual Harris name to William Henry Harris whos first son Larkin was born 12/25/1809. I would like to trace the actual Harris name farther back. (Task2)

I am only beginning my search, going on what Grandfather has begun but none of it is documented.

If you have any information on the Harris family of Pensicola Beach Florida or Tuscaloosa Alabama, please contact me.