This feels like an obvious criterium for me, but maybe I'm wrong: I don't think you should get a start for your country if you can't get a start for your club. I like Armstrong, but a lot of people on here talk about him starting in the upcoming matches and I just can't see how to justify that. He needs to actually play serious competitive football before he merits a start for Scotland.
Only exception to this for me at the moment is McBurnie, because I think he's being phased into the team still. I imagine he'll be starting for Sheffield Utd very soon. It doesn't seem like this is Armstrong's trajectory.