When it comes to what tartan to wear the only real "rule" is that you shouldn't wear restricted tartans. The convenient thing about that is that--barring an astronomically unlikely chance encounter with someone who has such a tartan on hand, needs money, and lacks the moral character to be trusted with such things--you just aren't going to get your hands on one of them.

Wile some will only wear their own tartan, by choice, there is nothing saying you can't wear any tartan on the market. Remember, even though many clans, septs, governments, regions, groups and companies claim(and sometimes actually DO own) tartans, the "Clan Tartan" was actually an invention of the weavers after Proscription was lifted. That's not to say that the symbolism is any less meaningful, just that it was an invention of the fashion industry in the 19th century. In a way, if you wear whatever tartan you like, you are being traditional--"Old School" too.

While centuries ago people wore any tartan, and sometimes several at once, the only "rule" I would ever advise someone to follow is to stick to one tartan at a time. That's just personal taste, but one that I think many people share.