It was already sort of covered, but a couple points to consider:
1) The American Civil War period (the mid 1800s) is not the American Colonial Period, which was pre-1800 (some wiggle room based on how long the American Revolutionare War, or I think the American Colonial War, or whatever they call it in the UK, and even the war of 1812 could be relevant or the French-Indian War from before the Revolution). These four wars each had very different motives behind each of them, and depending on how historically accurate you wish to be, technology and wartime tactics played a big part.
2) Guns were a large part of most of these wars, and it is not even accurate to lump them all under flintlock/basic firearms. Case in point, the gun improvements in accuracy and firing speed played a big factor in the reason why there were so many casualties in the American Civil War (alongside inept leadership and outdated tactics). Want a good example of this? Look up the battle if Gettysburg or the movie "The Last Samurai" for prove of just how lethal improvements in firearm technology can be (the film takes place JUST after the civil war).
3) The big one: Sexism and Racism were very prevalent factors in the American Colonial times and the American Civil War. Women were NOT (typically speaking, rare examples in history do exist) soldiers. They held duties in war time that were often making clothing or food for soldiers, caring for the sick or wounded, and we're (again generally speaking) not allowed to be present on the battlefield. There were however, a number of female spies and assassins who were often prostitutes.
The more pressing concern, however, is that of racism. It was a large part of society back then. Mixed races were NOT allowed even when among the soldiers. Assuming you are referring to the American Civil War, even the Irish were kept in separate regiments from the "American" soldiers, and were often underfunded or given the more dangerous assignments. The point is racism was a major factor in society back then, and should be considered if you want a historically accurate game.
Factoring non-human races into the equation would only complicate things more: are they actual elves/dwarves? Will your players be offended if all "french" people are elven and all "african" are half-orcs? (Hint: they likely will be, and probably should be). It is subject to much scrutiny and interpretation that may come off as offensive.
I'd sit down and determine *what* and *why* you wish to run a game in the period, as well as which period. Then determine how historically accurate you wish it to be. For example, is magic allowed? How will you handle gun proficiencies and even sword proficiencies in a culture where 10 year olds were taught to use rifles to help hunt, but not how to read, and basically no one except army officers fought with swords. What about religion? Etc.
I also point out, that an oddly diverse and inclusive treasure trove of a historical group of that historical period was down in the carribbean: pirates. There were a number of female sailors (some even captains) and freed slaves among pirate crews and they were suprisingly democratic. Look to the Assassins Creed Black Flag game for a great example of this, or that series in general for how to handle historical games.
