Great Being in disguise/Legendary individuals for the 14 most powerful Bastion members.
Toa/Hero figure for the 2nd generation of Bastion members.
Matoran/villager for the Bastion initiates.
Of course all the members can be any role, but due to the power their souls contain, it would be highly problematic if a lower ranking member just stepped in initially in a position of beings such as Makuta or Artakha without the necessary energy to sustain such a powerful body and to not get noticed due to their lack of experience. Any Bastion member currently in an infiltrated Biological Chronicles universe can move in power level if its deemed necessary and non-problematic.