In fact, to possess a certain type of troop, one generally needs to meet two conditions.
One condition is that you must become a hero of that aspect or type. As long as there is no rejection of the troops' beliefs and such, it's possible to win them over. This is similar to how players recruit followers of the same profession. Essentially, the player is the hero of that domain.
On the other hand, one must capture or recruit a hero of that aspect and utilize this hero to win over troops of that kind.
Li Yao had mastered engineering to meet the criteria of becoming such a hero.
In Li Yao's framework, his main force should be a Mechanical Army, with himself owning a Mechanical Legion. It's not that he wouldn't recruit heroes of this domain, but he Is the true master of it.
Moreover, in Li Yao's view, recruitment was too troublesome and costly.
