I have read part of the black historian's book who claimed his grand/greatfather fought for the confederacy. Parts of the book is available free on google. Read the first page or two. He isnt giving anyone a pass.
http://books.google....onepage&q&f=false
I am in no way trying to gloss over slavery but I am assuming that many of the traits of the folks of color and indigenous whites is not as cut and dried as the revisionists would leave you to believe. Having spent time in the deep south 12-15 years after the civil rights act passed with many of the same divisions that have festered for generations still very much at the surface I learned a few things. One being that a man regardless of color is proud, and he will fight strangers for his family, land and way of life even if the person/goverment/entity asserting the change claims it is for the better. So while it would surprise you to think that any male black in the south would fight on behalf of the confederacy it doesnt surprise me at all.
As for your cite that only lily liver'ed whites may serve, please explain General Stand Watie, The seminoles who also fought who were often half black, Creeks, choctaws and others. Trying to claim the south was all about rich white people is revisionism at its worst.