A Glengarry is neither use nor ornament in my book and as I did not serve in a Scottish regiment I am not too sure of the military/civilian protocols,when it comes to dicing on a Glengarry.I would think that as long as there were NO unit cap badges to go with it, all would be well.I stand to be corrected though.

Dicing on a balmoral,folklore notwithstanding,is easier ,you wear a diced bonnet or you don't.Up here undiced is the norm, although I am the exception.

Feathers look to me rather theatrical and out of place ,unless you are talking about eagle feathers and the people that are entitled to wear them.That looks fine to me,but all and sundry wearing non typical addornments in their bonnets should desist.Eagle feathers ,by the way,are not too difficult to obtain(legally)if you know where to look.