When it comes to uniforms I suspect they're costumers did they're homework, a rarity for early CW films. Most uniforms they have in the movie are similar to originals, and maybe sometimes miss-represented, (like in the charity ball scenes), but incorrect uniforms and accoutrements show up. Spanish-American War surplus haversacks, and uniforms on Union soldiers also the same surplus, and other stuff.
At the time CW veterans were still around in pretty large numbers, as was the UCV. There were companies that made "Confederate" uniforms for UCV members in Ohio and elsewhere, that are pretty much the same pattern as US Army 1880's sack coats, but in cadet gray and other colors. You can see these not-very-reminiscent-of-CW-pattern uniforms all over the place in Gone With the Wind.