I had Stewart ancestors myself in Kelso. They were butchers.
The only tartans they would have seen would have been if any Scottish army regiments passed through the town. It was only with the visit of King George IV to Scotland in, I think, 1822, that tartans became fashionable, and that had a lot to do with Sir Walter Scott trying to encourage a bit of colourful pageantry to make the king's visit memorable for him.
Lowlanders mostly wore quite dull brown or grey clothes, similar to what was worn in England, although the Scots favoured floppy flat caps, a bit like berets. In my lifetime (I'm 64) there has been an explosion in the popularity of tartans and kilts, and young men often get married in kilts these days, which was almost unknown in my younger days. People with non-Highland names like Smith, Brown, Watson etc. will look up books on clans and septs to discover which tartan they are "entitled" to wear, but it's really a load of nonsense! Americans love that kind of stuff, and the tartan shops in places like Edinburgh encourage them in their delusions.
For example, if your name is Watson, you will find that the clan books tell you that you belong to the Clan Buchanan, and you are "entitled" to wear the Buchanan tartan. But the Buchanans were a Gaelic-speaking clan in the Loch Lomond area in the west of Scotland, whereas my Watsons were fisherfolk on the east coast, in Fife, and had absolutely nothing to do with Buchanans or any other Highlanders at any point in their history.
Your interest is in Stewarts, and of course that's a fine Highland name, with an array of tartans for you to choose from. But it's also a common name all over the Lowlands, and my butcher Stewarts in Kelso are unlikely ever to have worn a tartan garment in their lives and certainly wouldn't have been seen dead wearing a kilt (there wasn't much love lost between Highlanders and Lowlanders).
Harry (Watson)