The units you receive (and the price you pay for them) are in the individual script files so they can be easily changed to fit an existing mod. Each client faction has several scripts so you'll be offered a variety of different client armies after you conquer them.

When you get them is determined by each script's trigger so you can have a look at those if you want to get the exact requirements.

The AI doesn't get units at present as the scripting requirements are more complicated - though that's obviously a prime avenue to be explored in any sequel.

If you'd like to learn more about scripting I've posted a beginner's guide on the other forum.