Sir Robert writes:
The official time period for the SCA is 400 AD to 1700 AD.
This is incorrect. The very first sentence one reads at www.sca.org is:
"The SCA is an international organization dedicated to researching and re-creating the arts and skills of pre-17th-century Europe."

Pre-seventeenth century means the year 1600 or earlier.

Robert also writes:
The persona might be say a Scot from 950AD and we don't really know for sure if they wore kilts or brats or what.
Yes, we can be sure that Scots did not wear kilts in 950 AD because the historical record shows that the kilt simply did not exist then. If a person is wearing a kilt and claiming to portray a Scottish Highlander from the tenth century, he is simply wrong. We don't know a whole lot about what Highlanders did wear in the tenth century, but as we can find no evidence of anything related to the kilt prior to the very end of the sixteenth century, it would be baseless to assume that this garment was worn six centuries earlier.

Will Prat writes:
Matt, for belted plaids to be worn by a whole group of Scottish mercs in 1594, they had to have been in use for a while. After all, they didn't just invent them for the job. Their being worn in 1594 certainly makes a date of origin of 1580 or a bit earlier likely and in fact lends credence to the descriptions ca 1575 that could be read either way.
Will, I appreciate what you are saying here, and agree with you to a certain extent. I think it highly unlikely that the group being described in 1594 as wearing belted plaids were the very first people to have worn that style. However, the fact remains that (as I have stated) the first recorded evidence of the belted plaid being worn is in 1594. It may likely have been worn prior to that, but any earlier date is just speculation. I think it is remarkable that of the many other sixteenth century accounts of Highland Dress that we do have, only one or two could possibly be imagined to describe the belted plaid. So it certainly wasn't common prior to the last decade of the sixteenth century.

Aye,
Matt