First there were only the vaegirs and the swadians.
The rhodoks split of from the Swadians, and adapted their army to fight against swadians.
The Nords came from another country across the ocean.
The Khergits came also from another country across the mountains.
It seems to me that the vaegirs and swadians differ much in troops, Swadians have heavy cavalry and crossbows mostly, while vaegirs have archers, footman, and light cavalry. Different.
Rhodoks mostly spears against the swadian heavy cavalry, and crossbowman, to counter attack swadian crossbows.
Nords solely infantry, complete different history culture etc.
Khergits solely cavalry, same as nords.
The history is certainly not very similar, and even if it is more realistic, I think that the troop trees should be altered, a different culture, so also different naming.