does the wearing of a Beret follow similar rules as wearing a Glengarry?
I'm thinking a Beret with my clan crest pin in place of the flash would look quite sharp.
As far as "rules", a beret is a hat just as much as a glen; as a matter of common courtesy and respect to your host, uncovering indoors is the only "rule" I can think of (I can provide several references to support the idea that this is a cultural norm in English-speaking societies; I'd dearly love to see a citation to support the notion that "The predicates were not to be applied in intersection but as modalities", et al.).
On the other hand, I think that balmorals or tams shape and look a bit better, as they have quite a bit more cloth than berets, but then the folks I know who wear berets are in the habit of soaking them in boiling water and wearing them until dry, so they shrink up quite a bit. Also, if you're shooting for a less "military" look than the glengarry, keep in mind that the only people outside of France who wear berets are military.
Besides that, berets aren't traditionally Scottish or Irish; they're French. If you're looking for a traditional bonnet; a tam, balmoral or caubeen would probably serve you better.