Hello,
I'm no expert on military uniforms but looking at your picture the two men seem to be too exact in their detail to be anything other than the real thing. The musket slingas are missing, which, if done for a stage production, I suspect would be there.
I think they are genuine soldiers from around 1860. The cap badges, as you say, are nothing like the Dumfries Volunteers' badge pictured. So either they are not of that regiment or the regiment changed cap badges prior to your picture. I suspect the former.
Not a great deal of help, I know but it may be of some use.
Kind regards
AJ