From the background story:
Might there be a caste system in Arrendis society?
Look into existing caste systems (e.g. in Hinduism) for inspiration?
Sub-castes? (e.g. soldiers include ordinary foot-soldiers, squad leaders and generals)
Castes might have physical differences due to selective breeding, genetic engineering, upbringing etc. For example, soldiers would be very muscular, scientists and rulers might be physically weaker but have greater mental powers. Possibility of large physical differences: Soldiers might have natural armoured carapaces, while some labourers might have extra limbs.
Sounds like all Arrendis are blindly obedient to their superiors, particularly workers and soldiers. They also cannot move between roles - a foot soldier is never trained as a leader and vice versa. This inflexibility might be a weakness other races could exploit to give them a chance against the stronger, better-equipped Arrendis.
How do the Arrendis reproduce? Are there distinct sexes, like humans? Are they hermaphrodite? Might they reproduce asexually? I kinda like the idea of them having larvae, which all look similar at birth, but which quickly differentiate into identifiable members of their caste as they mature (in response to pheromonal or environmental cues, as well as genetic differences?)
Members of some social classes (e.g. soldiers) might be produced by artificial means for consistent "quality".
Artificial sentience - do machines have equal status to Arrendis in their society? Artificial brains as advisors to the rulers and military leaders?
High-tech society - might cyborg-style technology be commonplace? Scientists and leaders could have neural enhancements, soldiers could have enhanced vision/neurally-interfaced weapons etc. Low-status individuals would probably not usually be worthy of such resources, but some workers might be physically integrated with tools or machinery.
Arrendis might have some interesting weapons. Maybe weapons using some of the more exotic subatomic particles, for a start? Have Alliance races managed to reverse-engineer any Arrendis technology?