Waitaminute you're saying that I can have command of more than 20 units in 1 stack? ...How come I was not aware of this before?? As far as I know, if I transport a 21st unit into the 20 unit stack, the exchange units window would come up. Or am I misinterpreting something?

I don't think checking the army composition has to do with re-ticking the controlled by AI option, but I'll recheck it. Thanks.