I think that part of this is traditional. Where the kilt sits natural waist, sporran etc. I would throw 21st century kilts in their as well, I talked to Howie and his creations are for people who want to wear the kilt, not tartan but in fabrics that one would make a suite out of. For these there are certainly guidelines and conventions that would certainly have Panache's hats fitting into nicely. Remember that Duncan on MOTG wore the kilt and it looked and worked as a practical working outfit.
The contemporary kilt utilikilt,freedom kilt etc. are bringing the kilt into the mainstream they would be not have the same guidelines, infact they may not have any other than what looks good. For those of not in the UK these companies are making kilt wearing more popular.
I think that there are only two rules.
PLEATS IN THE REAR
Basting stich removed.
Oh yea wear it with pride if you have the swish you should have the swagger