When do your wear a pair of Levi's? What do you wear with Levi's? When is it appropriate to wear/not wear Levi's?
Same with a kilt.
Most of this site is frequented/posted to/read by North Americans - 'most' - not all.
Unfortunately, if you ever visit the UK, especially Scotland, it is rare you will see anyone in a kilt. They are reserved for special occassions. Weddings, parties, New Year, Burns Night and street parades. Most times at these events, you will see guys in kilts with a certain degree of dress sense or formailty - in the case of weddings etc, normally advised by the kilt hire shop.
These are the only occassions you will see guys in kilts.
Forget moving out the city to small villages, countryside or farming communities. You will not find quaint old guys having conversations next to a stone wall, leaning on a shepherds crutch, wearing tweed, mismatched or even matched tartans - or balmorals. Scotland has moved into the 21st century. Most people living in the countryside wear gortex and drive Landrovers.
It is only in North America that there is a difinitive set of rules - based on 100 year old ideals.
(There isnothing wrong with that. Fashion evolves and diversifies)
If you want to see a real Scots guy wearing a kilt, google image search 'tartan army' - Scottish National Football Fans.
Try and work out what set of rules they follow.
The answer to your questions are simple. There are no rules, no codes, if you feel good wearing whatever combination you have, then it's got to be ok. Same with Levi's.