Trousers would work, but they would have to be of the non-frilly type and would be called trews, they would lso be a matching tartan to your troops.
.I would maybe recommend only giving them to command or ranked models too.
Trews came about from when the English were trying to stamp out any signs of Scottish patriotism, in an attempt to bring them to heel. The English banned the wearing of kilts, so the Scots cut their kilts up the middle and sewed them together to make trews, thereby obeying the law but keeping their scottish identity.
Kilts (genuine kilts) are made from heavy thick woollen materials and are notoriously itchy on the gluttus maximus if you sit for any length of time without underwear (atleast they are to me), which could help explain their legendarywrath in battle. Imagine stting in a chimera for a few hours with a very itchy bum and i'd bet you'd want to relieve your frustration by bashing the nearest thing to hand
lol.
http://www.highlandclans.co.uk/Mens_Tartan_Trousers.html
Hopefully this can give you ideas for a tartan to go with your troops.
Black devils is a cool name, and your minis look ace, however I think you need to start getting a little uniformity in the poses, or they will start to look more like a rabble of crazy men than a disciplined royal guard unit.
They maybe Scotsmen in space but if you lookinto military history you will find that the Scots were some of the most fearsome and disciplined troops in the British Army, for example the Blackwatch and the Cameron Highlanders.
However the poses you have are really cool and I like the use oh the middenheim mordheim sprue pieces.
Keep up the good work and.....
HAVE A FAB HOLIDAY WITH THE FAMILY, MAKE EVERYDAY A FUN ADVENTURE AND LOOK FORWARD
TO SEEING YUOU WHEN YOU GET BACK!!!