For a fascinating academic article on the Picts see www.clanntuirc.co.uk/JSNS/V9/Broderick%205.pdf which is George Broderick's article 'Pixti /Pexti, Picti? The name 'Picti' revisited' being from 'The Journal of Scottish Name Studies 9', 2015, 9-42.
Its key points are
Unprovable, of course...
On the silliness which is the tartan see this article http://www.theaustralian.com.au/news/world/the-times/tartan-tyrants-cant-tell-you-what-to-wear/news-story/319e8ab0a40e9c8c865b93ab4dd1d649 which originally appeared in 'The Times' but which was then copied in 'The Weekend Australian' in August 2016.
For a flavour of it consider 'The idea of ancient ownership of tartan is a myth, for the entire story of tartan is a glorious invention, cooked up by a couple of enterprising gay Victorian fashionistas from Egham in Surrey, popularised by a German prince and probably first worn by the Chinese.'
And 'Early travellers in the Scottish Highlands reported that while the locals wore clothing of different, and often intricately woven patterns, these did not necessarily denote allegiance to anyone or membership of any particular body, but depended on location, the availability of different dyes and above all personal taste. Some wore several different tartans at once.'
As is often said named tartan - such as the supposed Gunn tartan - is nothing more than tourist tat of the 19th century still being mainly by the tourists of today...
That link may work, or not, depending on the 'pay wall' aspect of the newspapers...