Pretty self-explanatory suggestion, really, so allow me to present some examples as to which kingdom would have which units / nationalities / minor cultures and where. Since we don't get much lore on other smaller cultures present in Calradia, most of these suggestions will be based on what we know from Warband [factions].
Vlandia
Sturgia
*
Khuzait Confederation (A confederacy would make more sense in this context, see: Cuman-Kipchak Cf.)
Other than the Khergit and the Karakhuzait I don't know any other distinct cultures or customs or anything that would make the Khuzait more diverse (the lore tell them they are very diverse, indeed a confederation or khanate of united tribes, but doesn't tell us how or where).
If we're to take inspiration from early-mid medieval Turkic states which the Khuzaits very much seem to be based or heavily inspired from, the Northern part of the khanate / confederation would be occupied by the Calradian equivalent of Kipchaks, and the south by Oghuz clans, make them rivals, maybe? The map size and lack of proper lore is too limiting to speculate this deep, but I'm sure it will be looked into.. sometime... maybe...
Vlandia
- Rhodoks, southern Vlandia (and please add some mountains down there). Rhodoks, unsurprisingly would be famous for their pikemen, crossbowmen and mercenaries(?) and their weak light cavalry (mostly for scouting I suppose).
- Swadians, rest of Vlandia. Vlandians would be famous of course for their knights, heavy infantry (and longbowmen please!).
Sturgia
- Nords, wherever they first settled when they came into Sturgia as mercenaries I guess? The lore doesn't specify that as far as I'm aware. I would guess the coastal cities and villages? Maybe a few villages populated only by Nords? Or even better, make them settle where their future kingdom would approx. be located, namely western Sturgia. They would be known for their huscarls and.. is that it? No seafaring implemented atm, can't give them longboats either. Hmm.
- (And I say Nords to mean 'pure Nords', who haven't completely assimilated with the rest of Sturgia)
- Vaegirs, eastern Sturgia*. Pretty decent cavalry, two-handed heavy inf, and decent archers.
*
(https://mountandblade.fandom.com/wiki/Sturgia)
Khuzait Confederation (A confederacy would make more sense in this context, see: Cuman-Kipchak Cf.)
Other than the Khergit and the Karakhuzait I don't know any other distinct cultures or customs or anything that would make the Khuzait more diverse (the lore tell them they are very diverse, indeed a confederation or khanate of united tribes, but doesn't tell us how or where).
If we're to take inspiration from early-mid medieval Turkic states which the Khuzaits very much seem to be based or heavily inspired from, the Northern part of the khanate / confederation would be occupied by the Calradian equivalent of Kipchaks, and the south by Oghuz clans, make them rivals, maybe? The map size and lack of proper lore is too limiting to speculate this deep, but I'm sure it will be looked into.. sometime... maybe...