This is more of a curiousity question about the way the game was designed. Brigades are made up of elements which are more or less regiment size. Most of the the Brigades you can build are really demi-brigades. That is, they contain only two infantry elements plus some attachements. Most brigades during the Civil War had at least four infantry regiments in them.
The second oddity is that they usually contain elements from other branches like cavalry and artillery. Only during the first few months of the war were such organizations raised, usually called Legions. The cavalry was the first thing removed and a little bit later the artillery was formed into separate organizations. Most of the weapon types shown in the elements for artillery included in infantry were discontinued by early 1862.
I haven't played into 1863/64 so I don't know whether the game phases these odd formations out or not.