This is a really neat idea!

It always seemed odd that you could conquer and occupy settlements, but never actually build units other than your own there. If ( when ) the Romans take over, say, Carthage... where do all the Elephants go ;)

This adds a neat little extra to the game by allowing you to utilise some of the special units available to other peoples, and I think this is a going to make for a much more interesting game. Should also provide an incentive to occupy and rule well, rather than slash and burn ;)