General Miles not an Anachronism
General Miles in this movie is obviously supposed to be General Nelson A. Miles, who decades later became the commanding general of the US Army..
But in 1875, the fictional date of Comanche, Nelson A. Miles was the Colonel of the Fifth US Infantry (Not the fifth Cavalry, the Fifth Infantry).
But it is not an anachronism for him to be called General in 1875. Miles had risen to the rank of Major General of United States Volunteers during the Civil War. Miles lost that rank when the US Volunteers were disbanded in 1865 and 1866, but did become a colonel in the United States Army.
Miles was also awarded the brevet ranks of brigadier general and major general in the United States Army. Brevet ranks were mostly honorary. But their perks included the right to be called by the Brevet rank and the right to wear the uniform of the brevet rank (when off duty).
So it was proper to call Colonel Nelson A. Miles "General Miles" in 1875, and for him to wear a major general's coat (and one made rather badly by the costumers, I think) in the last scene, the ceremonial treaty with the Comanche. I am not certain that would be allowed by regulations, but I'm certain the Comanche wouldn't know or object.
When I watched Comanche again last night I forgot to check whether Miles had a colonel's eagles or a general's stars in his shoulder strap's. Wearing general's stars while on duty would have been anachronistic in 1875.
What is really anachronistic about Miles in this movie is that Miles, born in 1839, turned 36 during 1875, but was portrayed by John Litel who was born in 1892 and who was 62 when Comanche was filmed in 1955.