With Kavadola's surrender and the sealing of his Divine Soul by Jack Clark, he was essentially considered one of their own.
Additionally, considering the connections between them and the Ancient Human Race and some other factors, Jack Clark and several other king-level figures briefly discussed and then ordered Kavadola to have his tribe migrate over.
As members of an alien tribe from the mythical world, even the weakest adult possessed Fifth Layer Heaven strength, the elite possessed Seventh Layer Heaven strength, and coupled with their Violent Dragon Alien Beast mounts and War Pattern Power, they were naturally born soldiers.
Once there would be sufficient food, breeding and raising a large number would make them a powerful vassal race for the Human Race.
As for settling their territory, that was even simpler, as the human areas in the mythical world were vast enough to allocate a space large enough to accommodate millions of the Violent Dragon alien tribe.