You can find quite a few similar threads, and the consensus has always been that as long as you don't represent yourself as a vet-or anything else you're not-no one cares. Don't wear ribbons or badges or medals you didn't earn (which is actually illegal), and ther rest is taken as a way of honoring the military and all vets.

As a Marine, I'd be tickled to see anyone in the Leatherneck tartan; it was designed by an acquaintance of mine.

As to bonnets, you can buy a regulation tam-o-shanter in olive drab wool on the open market (check out ebay and What Price Glory). I think they look good, and I wear one all the time. OR . . . you could get any black balmoral or glengarry.