By the way, this is more or less a balance issue, but it occurs to me that you don't always want your sovereign leading an army. Sun Ce, for example, with his Conqueror promotion. If he forms an army, he has no personal unit. With no personal unit, nobody can use the Conqueror promotion and though the overall army might have +1 strength or some such, this isn't strong enough a benefit to overcome the power gained from a Conqueror unit. There is a strong disincentive to use Sun Ce as an army leader versus a legion leader.
It's a little counter-intuitive to think that you might not want your strongest leaders at the head of the army.
I'd suggest giving the army leader a personal "honor guard" unit, so that they don't disappear as an entity.