With all the documentation on Harrison I find on Ancestry and none on Julian/Julan/Julin, also Julian was married with a child when war started and Harrison was single. I would say he didn't serve.
Since Julian died after 1920, his death certificate could have a stated wheter a Veteran or not.
For Harrison I found him in 2 Civil War Service databases, filed for Civil War Pension, listed in 1890 Special Veteran Schedule census. Julian none, and there are records for others of TN, Scott County, KY and Witley/Whitley County, so I would think he would be included if served.
As for age gaps of children, you have 1860 next child 1866, but 1860 census no children,(census taken July, married June) 1870 oldest is 9 and next is 5, then next is 1 yr old. 1880 census chldren years of birth are 1862, 1865, 1869, 1875, 1877, and twins 1880. So the gap between the first two children is not any longer that later children.