in the past only menos could use cero, and only arrancar could use sonido or advanced cero forms, menos couldn't even change the colour of their cero. but things have been changing.
the building of the grand city in hueco mundo, not only made it easy for hollows to find each other and feed to evolve, but ha finally brought hollows together to learn abilities.
cero is instinctive for menos, sonido had to be learnt by arrancar, but all hollows have the potential, it's just hollows, normally solitary creatures, never pool knowledge and advance in such ways, but now, knowledge is being stored and shared. basic techniques are being taught.
hollows are using cero, changing it's colour, utilising advanced forms like bala, and even using sonido. this coupled with the increased amounts of menos level hollow has led to soul society being pressed more.
This method of shared knowledge is almost like an academy, except it is not organized with teachers and the like, it is just random hollows passing it on easier through the city, till it becomes general knowledge, although hollows outside of the city would not have this information.
