Rob Roy is too late. He’s early modern rather than medieval.
You’re looking more at Llewellyn or Macbeth.
Check out this wiki article on the Welsh Marches or this one on Scotland in the High Middle Ages to get you started.
Specifically from a Film POV, visually the best recent one is Robert the Bruce (but that’s a bit later than I’d set it, although it fits with Nemedia being the Holy Roman Empire) or The Last Kingdom (which is English, and a little earlier, but right ballpark visually for the Welsh and Scots fighting around the Norman era).
Unfortunately, despite how narratively interesting (whatever else you can say about the Welsh, they do drama well) the period from the Norman invasion (1066) until the final defeat of Welsh independence (1283) is there’s very little visual media on it. Scottish history of Wars of Independence is more common because of the larger Scottish diaspora, particularly in the US.
In terms of culture and fighting styles, this is how it’d play them based on my understanding of Welsh and Scottish medieval history (something I’ve never studied in any real depth). You’re looking at small close knit principalities who’s major mode of warfare is raiding each other. Raids are not necessarily bloody, and often are as much a display of military prowess as they are actually fighting. They primarily fight on foot using Longbows or Spear and Shield (Swords are common, but they’re sidearms rather than the primary weapon of field combat: professional ‘soldiers’ will go armed with swords in towns, everyone will own a knife which is either a Dagger or Seax). Maille is common amongst the nobility and their companions but rarer amongst common soldiers. They have ponies moreso than warhorses, but rarely fight with them: they’re expensive, so risking them in battle is not as common as amongst the steppe peoples or the wealthier nations they border. Shieldwalls (or schiltron as they were known in Scotland) are still common, so you’d expect a squad on an open field to fight in that manner. They prize music and learning: their poetry is renowned as being beautiful, and a man who can’t sing isn’t going to get far socially (this is probably excused for foreigners, but they best at least appreciate good music).
In addition to the Castes from the Core book or the Scout book, I’d suggest looking at the Barbarian book. Skald and Law Speaker in particular fit. Bossonian culture is heavily influence by the ‘Viking’ cultures to their north, but civilised by interaction with the cultures to their south.