John Beaton of the 9th Va. writes in a letter home, "If you can conveniently get a black slouch hat, and send me, I would like it very much, these little caps are anything else but comfortable in rain or snowy weather the water runs down my neck".
If you look in the period uniform issue records there are two types of head gear listed, caps and hats. I don't know when the term "Kepi" became popular in this country but it is popular today and is a generic term for what in the 1860s was considered a "cap". Of course you must realize that there were different patterns of kepis or caps just as there were different styles of hats. No need to over complicate the issue.