Note the suspender buttons. Issue trousers of the 1880s would not have those, (Could have been added of course) also the big watch pocket is suspicious, and the trousers may even be two different shades. See also the creases indicating shelf storage. (the army did not become enamoured of creases until after WW1) Definatly done in a studio, and most studios of the day even had uniform items for those who wanted it. I just don't see much active duty here I'm afraid. Wool pants and pullover flannel were pretty universal at the time.
Take another look at the guy on the left. Thats a Mexican Double loop holster, with a DA revolver! Probably a Colt 1878. The holster seems to indicate this was taken in the 1890s, probably close to 1900 give or take a few years.