I'm looking for figures to do this war in 6mm. I think Confederate type infantry will do as most Spanish infantry, with some British colonial troops as some of the better dressed units with sun helmets.

Problem is, what would be good for US troops of this period? Suggestions please.

I'd go to civil war infatry in hats (not Kepis) and use those. Boers could work too, particularly for units like the rough riders.

Dark blue tops and light blue pants were used, as well as various combinations of Khakhi.

So, ACW figures for both sides? Just need a different paint job...

It could work...

CR10 and 11, Natal native horse might also work well for rough riders, particularly with them having slouch hats and ammo belt over the shoulder.

Are there any artilllery crews in slouch hats?

I looked at the Boer War and a bit surprised to see there are no Boer artillery,

Artillerymen of the time were, infact, issued sun helmets, though many drawing show then in hats.  It would not be incorrect, however, in the spirit of proxys, I would use british colonial artillery, paint the uniforms us blue and the sun helmets white. (the Osprey book on the Spanish American War shows this uniform in a couple of plates).