I would agree that it was most unlikely, but there were only two Nellis families that I can account for in Pennsylvania and the other one, not connected to the 1709 immigrants, had very, very few males, and apprear to have died out. I know that there was no Thomas Nellis seeking a pension for the Civil War service. Of the 1709 family, there were only about a dozen that had Thomas as a given name. That further narrows the field. The War lasted until 1865, and they did have children as young as 10 enlisted. I would agree this is a very long shot, but, if a 10 year old enlisted, he more than likely came from a troubled family. The 71st was aka as the California unit, I have only found a half dozen Nellis men following the gold rush, none from PA.