This would be a nice addition. I would like to add one part - the soldiers that you capture could join your own army if you pay, say, 30% of the normal price. You would need free population too, but the cost reduction in itself would be a nice incentive.
I especially like your idea about a new hero skill, and the different capture rates for each soldier. I would have to say that the swordsmen capture rate should be low, since nearly every army has a large number of them. Perhaps knights, swordbretheren, and royal guard could have the highest rates because they are more "royal" units and would be more likely to capture rather than kill in real life.
This would be a great idea - it could even spawn a new building, the "Stockades", the higher level that you have of, the more prisoners you could hold. In fact, if the player captured at least 50 of some soldiers, they could sell them on auction in the "Stockades" as well, albeit at a lower price, say 5 gold for infantry. Throught the game, unless you pay to make them your "own" troops, they could keep the name of their original owner. This way, you could buy back your own troops on auction from people who may not have even captured them.
This may be too much, but what about capturing heroes? Just something to think about....
Overall, this is an amazing idea, and I hope my additions would improve it. I hope that moderators and developers see this and possibly implement it in the future.