So, I have assimilated several minor factions, yet I still cannot produce their units. Can somebody please explain what I am missing?

You have probably just pacified them. To assimilate you have to go to your empire screen and right after image of your leader there should be option to assimilate.

Later there's a tech that allows you to assimilate a second minor faction. I usually assimilate a minor faction that has the most villages inside of my empire, so I will get the most out of the bonuses that they provide (assuming it's the bonus that I actually want).



Cheers.