Good questions. In general, I tell people that there is no such thing as a "casual" kilt or a "formal" kilt, but what makes a kilt formal or casual is how you accessorize it.

So I would say that a great kilt can be perfectly fine for formal events as long as you accessorize it accordingly. That being said, here is some advice.

If you plan on wearing your great kilt to formal events, get quality cloth. A lot of people get great kilts made from tartan fabric that is either a woolen blend, not wool at all, or of a light weight. If all you are doing is wearing it to a festival, or a rondevous, or something like that, this isn't a problem. But if you want to look your best, splurge for the real deal and get a good kilting weight worsted wool. It will look and hang better.

Also, I tell people who wear the great kilt, usually, when you are putting it on, not to take great care in the pleating and try and mimic a modern tailored kilt. This is a mistake a lot of people make. It is more accurate for us to think of the great kilt as being loosely gathered instead of pleated. You'll read as much in my Early Highland Dress. But if you are going to a formal event, please feel free to ignore this otherwise sound advice. If the Highlander of Old was getting dussied up for a nice occasion, he certainly would have paid more care to how his feilidh was arranged, so why shouldn't you.

Lastly, remember that the great kilt is an historical peice of clothing. You need to decide how far you want to carry that. Do you want all your accessories to be historic as well? If so, you need to do some research and try to find sources for historic "formal wear" because much of what you see on the market just won't cut it. On the other hand, if you are wearing your great kilt merely as an alternative to the modern tailored kilt, feel free to accessorize it with modern formal wear accessories.

Aye,
Matt