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. 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. 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. In hot climates it is clearly superfluous and its use unnecessarily ostentatious.
Informal wear is something else and there are a wide variety of options available.
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.