The 1500s are more of the early Renaissance rather than the Middle Ages. However, Ranald Nicholson's Scotland: The later Middle Ages may be of some help.The main language of Scotland at the time was Scots, which was spoken throughout the country and Gaelic which was spoken mostly in the Highlands and some of the Islands.
The history of Scotland in the 16th century is very long and would take a lot of study. It is still the reign of the Stuart monarchs and will be for the whole of the century. In 1500 itself James IV, Scotland's tragic warrior king is on the throne and will be until the battle of Flodden in 1513.
In terms of what the people would have looked like, the dress was no different to that of the people of England.
The Robertsons were Lowlanders and so would not have been part of a 'Clan', I can't speak for the Duncans. The dress of the two would have been the same - they would not have worn a great kilt or tartan of any description as these won't be seen, and then in few numbers, until much later in the century.
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