A possible way to approach what is and what is not acceptable is to equate the various forms of Highland attire with its non-Scottish counterpart. The kilt has a range of uses, from full-dress evening attire through the equivalent of a suit to informal working dress and should be seen as a Scottish version of ordinary modern daily wear.
Gala evening occasions are one of the few times it is acceptable to wear everything you possess - if that is your wish; dirk, powderhorn, brooch and plaid - although the wearing of this last particular article of dress has to all extent disappeared in Scotland and would be viewed somewhat askance if seen at one of the Gathering Balls. One thing seen overseas and never in Scotland at any time to my knowledge is the wearing of bonnets indoors which seems to be prevalent, possibly to allow the display of feathers whicha ssumes a quite disproportionate importance overseas. But would you wear a hat indoors with your tuxedo'! And would you wear abroad sword at the Office Party?
For the formal and semi formal event by day when you might expect to wear some form of suit, then a tweed jacket and tie with the kilt would be the norm. Don't mix evening wear with day wear; an ordinary colored tie is not correct worn with an evening jacket by day -not unless you are in the habit of gong to your office wearing a tuxedo and for the same reason your sporran should not be an ornamental evening one but something of more restrained pattern.
And do Not attempt to add to your day attire with a flyp laid and broach; the fly plaid as sometimes worn in the eveningis a purely ornamental article of evening dress. if you wish to carry a plaid by day it is the full and original length of tartan which is carried over the shoulder which can be used as it was in former days as a shield against the weather. In hot climates it is clearly superfluous and its use unnecessarily ostentatious. And the carrying of a broadsword or wearing of a dirk or any other weapon at normal, day events - unless you are in the habit of going about your daily life similarly armed - is not to be recommended.
Informal wear is something else and there are a wide variety of options available. Discard your jacket, keep your tie and rollup your sleeves if you wish to retain some vestige of formality; otherwise an open neck sleeveless shirt is obviously perfectly acceptable.
The difference in climate between Scotland and many parts of the world in which Highland dress is now worn is of course a problem. It is not always realized that in spite of the well ventilated nature of the kilt that it is very much a cold-weather garment which is not suited to heat and which our ancestors usually discarded as soon as they could, even in military uniform, in favor of any available garb better suited to the climate. The use of lighter weight materials is perfectly permissible and is the obvious answer.
As regards headgear, Highland regiments have worn a variety of hats with the kilt in both hot and cold climates including various forms of tropical helmet, wide brimmed slouch hats and fur hats, Russian style. So there is certainly a precedent for other things apart from the bonnet. the civilian bonnet with military feather hackles.
Many people today are wearing the kilt far too long, right down over the kneecap so that virtually no bare leg appears between the hem of the kilt and the top of the stocking. The result is frequently grotesque. The edge of the kilt should cut the kneecap, not hang below it. On a rather delicate subject, if you are wearing the kilt in the traditional way you Must know how to sit in it correctly. during the recent overseas visit of a prominent Scottish functionary who was obviously wearing a kilt for the first time in honor of this trip and who on a succession of platforms revealed to those present views of Scotland which are not usually on the tourist itinerary.
Taste is a very personal thing but as a general rule, you can seldom go wrong by under statement. Highland dress has real glamourand romance and you do not have to go over the top in any way in order to make the best of it. By far the best book on the subject of wearing Highland Dress is :"So You're Going To Wear The Kilt" by J. C. "Scotty" Thompson. It is currently available in print and all who wear the kilt, whether indigenous to Scotland or not, should buy a copy if they seek guidance.
Once again, the important thing
to stress is that we are talking of National Dress not Fancy Dress and we
must be very carefulnot to bring the former into disrepute by allowing it
to degenerate into the latter.