Eureka!

This is a version of said article. I quote:

No specific Canadian unit was being portrayed in the film, though the main characters were said to be from Montana. One line in the film, a voiceover, informs the audience that the three brothers portrayed by Aidan Quinn, Brad Pitt and Henry Thomas went to Calgary to enlist. By coincidence, in 1993 the real-life Calgary Highlanders had just adoped a sporran badge that was identical to the collar badge adopted by their predecessor unit, the Tenth Battalion, CEF, which was formed from a Calgary regiment in 1914. It was therefore suggested that some of these badges be provided to the principal actors and extras. The time period for the film was early 1915, and research indicated the C over 10 badges that were adopted may or not have been in general issue at that time; but the change was made. Henry Thomas shows up on film clearly wearing the Tenth Battalion badges.
The Highland Company, wearing Cameron of Erracht tartan, found that one company of the 16th Battalion (Canadian Scottish) did in fact come from the Cameron Highlanders of Winnipeg.
--http://www.canuck.freeservers.com/legends.htm

And yes, you are correct about the Calgary Highlanders being raised after the war, however the author seems to imply that the Highland Company's presence in the film was a tributes of sorts to them and Cameron Highlanders of Winnepeg.

Regards,

Todd