When capturing not native city,it provides about 20gold income, but need at least 30gold to pay for garison (because rebelling).
So its not profitable to keep not native cities, but historicaly is well known, that conquerors were receiving big profit from conquared cities.
So I think You should think about this disbalance.

Is there any way to find home city of units? I cant find.
It will be good feature

When you have a unit selected, you can find their home cities by pressing the button called "Select Home City" in the lower right corner of the screen. The keyboard shortcut for that is pressing "3" on the Numpad.

In the gold version you can now also change the home city of a unit by holding down the right mouse button over a different city and selecting Change Home City on the popup circle.