Dale Seago wrote: “So if someone jumped on you about wearing a particular tartan because it ‘belongs to’ Clan X you could just reply, ‘Neener neener, doesn’t either, it’s 1690 and Clan X doesn't have a tartan yet.’ ”

Somehow this doesn’t ring true for me. And I am not entirely happy with Todd’s (Cajunscot’s) approach either – “. . . perhaps it's better to wear a non-named tartan”.
To my mind, the problem with a non-named tartan is that once is woven, it has an existence, and logically should have a name. Simply insisting that it is nameless seems rather silly to me.
Certainly one should strive to wear garments that fit the period – the breacan feile rather than the filibeg – and to use colours that were actually produced in those times.
But one is still stuck with the question: What tartans were worn in those days? And the answer is: We don’t know.
So anything one comes up with is an invention.
And, as Zardoz remarks: “As long as you don't mistake the ‘renaissance faire’ for a ‘period-correct renaissance era re-enactment’ you may ‘get away’ with a tailored kilt and lots of other ‘anachronistic’ things as part of your costume.

Perhaps in closing I should quote Yul Brynner as the King of Siam: “Is a puzzlement!”
Regarfds,
Mike