theThe goblins in my world are classical fantasy goblins. They aren't too smart, have primitive technology, live in tribes, and use tamed boars, wolves, and other wild animals. They usually have a boss, who runs the show because he is so nasty, convincing, strong or intelligent that the others respect him. The goblins raid or to extort income from nearby communities. They like to consider themselves great rulers.
However, I don't want them to build big and complex empires. Nothing more complex or closer to a state than their extortion scheme territory. Additionally, they are supposed to be extremely unreliable allies, whether they forge an alliance with their kin or other races. Additionally, their tribes are supposed to remain small, never bigger than 500 goblins.
The best explanation I could come up with is based on Dunbar's Number. For them it is larger, the 500 goblins per tribe I mentioned, but they are unable to trust someone who they don't know personally. They can develop trust in new people, but they tend to act paranoid and violent around those who they don't consider members of their tribe.
Is this a good and believable way to achieve my goal here? Are there alternative/better approaches?