In ch.294, tesla asks Nnoitra why he picks on nel so much and not the other espadas. Nnoitra replies saying he hates her b/c shes a female & is a higher rank than him, a male. There are only 2 female espadas and since Nnoitra is #5, Halibel has to be lower correct? He doesn't pick on Halibel (or at least there is no proof of it at the moment) because, although shes female, she is not of higher rank.The only lower rank unknown at the moment is #7. Of course Halibel is also higher than male arrancar as well but it seems like Nnoitra only cares if they are above his level, not others.