@Ramen I was reading some things on the Szayel and I just noticed some things.
I was unaware of the whole "Cero Espada" thing because it's from a novel, but after reading through it I found figured something.
Yylfordt Granz is the single soul that Szayel separated from his body that made him go from Vasto Lorde to Adjuchas (from Espada 0 to Privaron Espada), but Yylfordt Granz is "Arrancar Quince", "Arrancar 15".
Espadas are numbered from 1 to 10 in a ranking of power, while former Espadas are just randomly numbered 100+ numbers in order of demotion. On the other hand, normal Arrancars from Aizen's army are numbered from 11 to 99 in strict order of creation/joining, which means Yylfordt was one of the very first Arrancars to join Aizen's army. He was not necessarily the fourth person to join, since stronger members might have directly joined as Espadas, but he should still be among the first 10 or 20 at most. Yylfordt did not exist while Szayel was number 0, which means that, despite Szayel having been indeed a Vasto Lorde in the past and having casted away some of his power to stop having killing urges and better center his efforts in science, he was still only number 0 while his competition was only a handful 10 or 20 other arrancars and at least before Grimmjow and others joined.