The Mary Pettus who married Thomas Brown was the daughter, born 6 November 1746, of Thomas Pettus and his wife Amey Walker of Lunenburg County, Virginia (bible record). Thomas was the son, born on 25 December 1712, of John Pettus and his wife Ann (bible record). Although several theories have been put forth over the past century on the identities of John and Ann Pettus, no one (to my knowledge) has yet offered any solid evidence confirming a particular theory. Thus, the theories really lie in the realm of speculation, although some may be more plausible than others.
I don't recall having connected John Pettus (mentioned above) to Col. Thomas Pettus in any of my past GenForum postings, since I have long believed that the line of descent from Col. Pettus probably ended in 1700 upon the death of his unmarried granddaughter, Elizabeth Pettus. I have stated that opinion in past messages that I posted on GenForum.
With regards to a John Pettus who married (1) Mary Minor and (2) Mary Waters, I think that you have in mind John Pettus, sheriff of Louisa County, who died in 1770 (will probated in Louisa County); however, I have found nothing on record that proves either marriage. Of course, it is possible that John did marry one or both of those ladies, but aside from the fact that he gave some of his children the Waters name, I have no further supporting evidence of a Pettus-Waters marriage.
One thing is certain: John Pettus, sheriff, was not the father of Mary Pettus who married Thomas Brown. John did have a daughter Mary Waters Pettus (will), but she married Charles Blount of Louisa County (marriage bond, 1 December 1787).
Similarities in the names of the above John's children and those of Thomas Pettus of Lunenburg indicate that those men were brothers, but even that conclusion is somewhat speculative.
I have primary documentary evidence for the ancestral line of Gov. John Jones Pettus of Mississippi and his brother Gen. Edmund Winston Pettus of Selma, Alabama, back to Stephen Pettus of Hanover County and his wife Mary Dabney. Using your notation, I have Stephen Pettus of Hanover County, Virginia, m. Mary [?] Dabney, daughter of Capt. George Dabney of King William County, Virginia> Dabney Pettus of Hanover County, Henrico County, and Charlotte County, all in Virginia, m. Elizabeth Rhoades of Louisa County, Virginia> John Pettus of Fluvanna County, Virginia, m. [--?--]> John J. Pettus of Henrico County and Fluvanna County, Virginia, m. Alice Taylor Winston of Hanover County, Virginia> John J. Pettus, Edmund Winston Pettus, and six other children.
I am not aware of any connection between the Pettus family and the Taliaferro families; however, there may have been some connection between the Pettus and Grymes families. The latter possibility is indicated by the name of William Grymes Pettus of Lunenburg County and St. Louis, Missouri. Thomas Pettus of Lunenburg (b. 1712) had a son Thomas who lived across the Meherrin River in Mecklenburg County. Thomas of Mecklenburg. Thomas Pettus of Mecklenburg County had a son William who was father of William Grymes Pettus.
As you can see, the genealogy of the early generations of Pettuses in Virginia has been the subject of much speculation and uncertainty, much of which seems to be ill-founded and probably wrong. Unless new evidence comes to light, some problems may never be resolved with high confidence. My forthcoming book has a wide-ranging discussion of these problems and what appears to me to be the most plausible solutions.