Also I think some units may get special abilities, Saw a Hammer Unit with a special attack, so there may be units with a single spell attack.
In the Beta when you got a Champion to join you there was an Imbue spell that transfered Essence from your Sov, to the other so they can cast spells you have researched.
Hope it helps, the game has some very deep interactions in some areas and I expect the day0 patch to really beef up the AI a bit.
Enjoy the game and do not forget to Eat and Drink
Lee