From my own research, by definition, if you had a direct family clan link, you could wear the Clan tartan excluding the Chieftans and his DIRECT family tartan.
If you were a bondsman, you could also wear it to show your vassalage.
If you sort protection from the Clan, you were also entitled to wear it as you would be expected to pay tithes/support the standing force etc.

With records only recently being available to all via the various ancestry websites, it is easier to trace those links except for the vassalage, but due to many scribes who performed the various census over the years recording names phonetically, and if the person had any speech impediment or unfamiliar/strong accent, plus with people changing spellings themselves for various reasons, these things should only be taken with a certain amount of surety.

Add to that the fact that buildings burnt down, flooded etc it can make it even more suspect as records were either lost completely or rebuilt from other sources who had the same issues, it gets even harder.

Also when you add transaltions of foreign names of peopel who fled persecution, etc, into the mix...

My brother 'researched' a VERY linear and short family tree and 'proved':
  1. We have no Scottish connection;
  2. We have no Welsh connection;
  3. The oral lore of the family was 'made up';
  4. Our father was adopted (news to his elder brother and uncle...)
  5. He has remarried 8 times; and
  6. He has 12 other children aside from the 6 of us...
  7. A friend who was a professional genealogist didn't know what he was doing when he traced our family as a wedding present.

If you can find a connection, however looesly, rejoice in it. If you prefer one plaid over another, where it.
Aside from the Chiefs personal plaid and corporate plaid, you can in reality wear pretty much any plaid you like. Only those who are plaid Gollums woudl challenge you and even if its not your direct family plaid, very few would complain but would appreicate the honour you do by helping keep the plaid alive.

A lot of people wear plaid because of fashion - but what 'right' do they have aside from the fact that they paid ridiculous amounst of money to be fashionable.

Wear what you feel comfortable wearing and rejoice in your 'melting pot' heritage. Steel is stronger and more durable than iron, afterall.