Even though many of them are actually STANDING, I believe the more frequent usage is to SIT for a portrait, not pose for it ( one poses for photos, sure, but paintings take longer). I wouldn't attach too much significance to modern connotations of POSE when discussing these images.

And, I believe one is PORTRAYED in a portrait.

I think the easy test is to look at these gentlemen's outfits and their job descriptions. How many gamekeepers would take to the Scottish countryside in such dainty footwear when sturdy boots were available?

Many of these men traveled to be painted- sometimes to Inverness, sometimes to Edinburgh- At the very least, they would have worn "town clothes"...