Hey Kael et al,
One of my favorite civs in this game is the Hippus and the concept that they are generally a Mercenary civ. I generally dont get the feel that they truly are such. I have been banging my head about this. They are "known" to sell their services to the highest bidder, but in actual game play they do not. I was thinking more about this in the TAM mod when what you did came to mind. The Hippus palace grants them one horse unit so that they can build their unique units and add to their flavor. Why not add another "resource" to their palace. This resource would only be allowed by the Hippus and would give them the ability to sell off their "services to the highest bidder". The resource would then allow the other civ to build a unique unit call the Hippus Mercenary. This would also allow the Hippus to build this unit when they have "no contracts" to fill. As the game advances so will the strength of this unit (ie by upgrading to a stronger type as techs and money allow).
This way we can keep the existing mercenary system and add the Flavor to the Hippus to go with this back story...