Due to their immense power, Espada of Rank 4 or higher are forbidden from releasing their Zanpakutō within Las Noches. It is later justified by the battle between Ulquiorra and Ichigo, as their fight above the dome resulted in Las Noches losing a good portion of its domed roof, as well as segments of the five towers.
The Gran Rey Cero is so overwhelming, that one of the only two rules the Espada have,is that, that they must never use it underneath the dome of Las Noches as the sheer force of just using can cause Las Noches to be destroyed.
Neliel is a former Espada and she is not as powerful as the current Espada ranks.
Grimmjow is blunt, rude, sarcastic, witty, and quite sadistic, revealing a psychotic grin whenever he becomes excited, much like Kenpachi Zaraki. He displays little respect for authority and says whatever is on his mind, regardless of whether or not it is appropriate. This attitude leads to conflicts. (Also, Grimmjow fired the gran rey cero outside of Las Noches palace and thus was not violating the rule.)