@bakauguu There are multiple clans in a single tribe and Yulian took members from all of them independently. Remember, that the different tribes in this setting are pretty much independent countries with their own armies. A soldier from Finland for example wouldn't just jump over to join an independent taskforce in USSR, for example, especially during a time of war. Without approval or treaties from the other tribes, taking warriors from them would easily be considered as treason or an act of war.
While the time of Red Storm broadening it's influence will most likely come, it has to first get foothold and compliance for it.