I read somewhere that among those of Custer killed were two ex confederates...and a deserter from the USMC!
I know of Southeners in the 7th, like Benteen, but from the top of my head I can`t remember any former CSA soldiers besides the one mentioned? Do you have names? i know some had relatives and fathers who fought for the Confederacy. But then again, weren`t many in the Frontier Army former Confederate soldiers? Or was this maybe more common straight after the ACW? As for deserters, it seems like it was pretty common to take a break and sign up again later, with the Cavalry being a real hot spot for desertion.
The 1876 campaign occurred 11 years after the end of the war on slavery. SO any veterans of that war would be middle management. The enlisted pay was lower than a general laborer. The officer side was more user friendly to ACW vets, but only union.
Bottom line. There were some former CSA members at LBH, but they were not a significant element of the US Army in 1876. Fred Wagner's participants book identifies many. Since most used fake names, there could be more.
The majority of the 7th joined the army after ACW ended.