Hi - since no one else has responded, I will. It sounds like you have a pretty good start with grandparents and a marriage license for great grandparents. I would suggest you drop a few dollars into a pay-as-you-go account with
www.scotlandspeople.com and go from there. That is about the only place where you will be able to find Scottish birth, marriage and death certificates, as well as Scotland censuses.
Death certs for grt grandparents, as well as their marriage cert, should give the names of their parents which will get you back another generation.
When you complete these efforts, come back for more advice. You should be able to get family back to 1840-1850 with few difficulties.
Nick