Roy,
First off, 19th century uniform regulations, by our modern standards, are a little more vague than we are used to. Since few medals were issued to American military personnel until well into the 20th century there was little need to establish criteria as to what could or could not be worn, or even how they were worn. Many medals that are seen being worn in various photographs are more often than not those of a veteran organization, to include attending a significant reunion or gathering, and other than being somewhere in the area of the left breast, there wasn't much consistency.
As a general rule, medals/decorations/awards were worn on formal occasions such as parades, balls, and the like. They would have been worn when the occasion called for it, regardless of where you were posted. Obviously, many wore to them when they had their portraits made. Some countries allowed them to be worn at other times, but since you are protraying an American major then you should follow US Army customs.
Within the GAF, as we discussied on the earlier thread you started, and assuming you want to be historically correct, you could wear appropriate campaign awards (the been there, done that type awards) that your persona could have earned a right to wear and been expected to wear at the time you are portraying. You should avoid the Civil War or Indian War campaign medals since they didn't come out until the 20th century, as well as any others of similar nature as pointed out by St George.
Within SASS there is no restriction on what you wear, or how you wear it. As I pointed out previously, I wear my SASS badge suspended from a medal ribbon when attending SASS events. This summer I went on a trip to Gettysburg with about 24 SASS folks. At dinner one night I wore my SASS "Medal", but most nights I wore my GAF medal. I did the same at this year's SASS convention. When I go to NCOWS events, I only wear my GAF medal since they place a greater emphasis on historical accuracy.
The GAF is not a reenactment or living history group, though we have members who engage in both activities. We were created by CAS shooters who wanted to wear a uniform when they shot, but now we include not only shooters, but reenactors and history enthusiasts, some who don't shoot at all. We do not REQUIRE anyone to purchase or wear a uniform, though we ENCOURAGE our members to do so. I'm pleased to say that many of our members do acquire and wear a uniform, and as a whole we lean more toward the historically authentic look than the Hollywood image.
In summary, if your persona was a soldier of fortune and was in the right place at the right time to be awarded a campaign medal, then you can wear it. I doubt anyone would be fooled into thinking that you actually served in the Crimea or the Zulu Wars any more than some of us had been in the War with Mexico.
US Scout
GAF, Commanding