Just like shinigami Arrancars have their ranking system too. Now if you are an espada, yes that means you are stronger then any other type of arrancar and cannot be defeated by them but if its another arrancar, because of your level doesnot make you invincible, you can be defetead. Below are the diffrent ranks for arrancar. there will also be limited spots for these as well.

Espada
10 Positions

Privaron Espada
10 Positions

Fracción
Unlimited

Números
Unlimited

Hollows (doubt anybody wants to be an hollow lol)