Unlike soul society, the hollows are not a military force, they do not have squads and ranks, save what their evolution gives them. but they do have structure.
at the top is the leader, few ever see him/her, but they know he exists.
next we have the triad, his/her favoured warriors, and apparently friends. all of them vasto lord arrancar, and all very powerful, they make sure things in Hueco mundo run smoothly.
Next are the espada, they are the ten greatest arrancar, save the triad and the leader. ecah one has a few favoured hollows, that they call their fraccion, they are friends, or simply strong hollows. they are the closest thing to a shinigami squad.
next are the vasto lord arrancar, who aren't espada.
then the vasto lord hollows
then the adjucha arrancar.
then the adjucha hollows
then the gillians
then the hollows.
the last few are simply evolutionary ranks, they command, simply because they are strong enough to.
