User blog comment:Silver-Haired Seireitou/Sake Time with Sei - Can Arrancar Use Kidō?/@comment-1493741-20140413171315

All implications (To me, anyway) are that it is an art, like fencing or dancing, one that can be taught as long as you have high enough Reiryoku for the spells and the knowledge on how to conjure them. Even if Hollow's lack something that Shinigami possess that is key to the process, then studying it also grants the possibility of it being re-engineered to suit a Hollow's differences. At this point, I'd consider a Cero to be the same as Kidou, Cero is Reiryoku molded by raw power, Kidou is Reiryoku molded by mental control to imbue it with a purpose, hence barriers, traps and the like.