Further: is there any record of ex-confederates serving in the Spanish-American War?

There were a variety of attitudes among former Confederates regarding reintegration with the United States. On one end of the spectrum, you have James Longstreet, who became a Republican (and a Catholic!), and enjoyed a successful post war political career in the U.S. government, for which he became widely reviled in the Reconstruction South.

Somewhat less enthusiastic about the new order were more situational reconciliationists, like Robert E. Lee and Joseph Wheeler. They accepted the U.S. as their legitimate government, took the loyalty oath, and at least in Lee's case, petitioned for a pardon.

Lee is an interesting case here, because while outwardly he was spotless in his advocacy for reconciliation, and publicly expressed no animosity towards the North, inwardly, he retained some intense bitterness from the war. He resented Union military actions he believed transgressed the laws and customs of war, the personal insult of the National Cemetery at Arlington, and the 'savage and brutal policy' of Emancipation, which he believed threatened the South's social system with destruction.

After the war, he opposed the disenfranchisement of former Confederates, admitted to no treason, supported states' rights, and opposed the extension of franchise to African Americans, though he did support the black public school system. He remained personally opposed to the Republican Party; when his old friend James Longstreet requested his endorsement, Lee politely but firmly responded that he did not believe his party's agenda was in the best interest of the country. I think general Joseph Wheeler would fit into this camp, since while he remained a Democrat and maintained a plantation, he fought for the U.S. in the Spanish American War.

And then you have people like Jubal Early, the archetypal Unreconstructed rebel (and apparently an 'imaginative curser', though sadly our sources don't have examples). Rather than surrender to the Union armies, he escaped to Texas in hopes of joining remaining Confederate armies, and thereafter to Mexico, Cuba, and Canada. He never requested a pardon, and became influential as one of the originators of the Lost Cause conception of the war. He died in March 1894.

I say this to provide context for the following quote, from Longstreet to Wheeler.

"Joe, I hope the Almighty God takes me before he does you, for I want to be within the gates of hell to hear Jubal Early cuss you in the blue uniform."