How common is the wear of a Plaid that is a different tartan than the Klt?

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I would think it depends on what type of plaid. A Fly Plaid (which is meant to simulate the excess fabric of a great kilt draped over the shoulder, and would thus be expected to be of the same tartan as the kilt itself)... or a Shepherd's Plaid/Laird's Plaid, which is generally folder over the shoulder and intended to be "at the ready" for more utilitarian uses such as to offer a lady to use as a wrap when it's cold, to place on the ground as a picnic blanket, and so on. These are free of the requirement to "appear" as extensions of the fabric of the kilt, and thus are often seen in a non-matching pattern. That is not to say that one would generally pick another clan tartan, etc. But perhaps a given clan's hunting tartan to go with the same clan's principle tartan, or a more generic simple check pattern, etc., such as the one shown here: http://www.scotweb.co.uk/products/shepherds-plaid-men/ What kind of plaid are you asking about?