One of the greatest features of Galactic Civilizations is the ability to design your ships.  Should a carrier be a ship later in the tech tree or a module like for the Troop Mod to add to your ship?  If you make a Hangar Module, you can design your ship to include this feature.  Depending on the size of the ship, you can add more hangar modules.  The hangar module should provide a number of small craft depending on the size of the ship.  Even Enterprise, a cruiser size vessel, had a hangar module for shuttle craft.  A hangar module would allow medium ships to have one or two small craft and a large ship to have possibly two or three hangars to allow a couple dozen fighters like Battlestar Galactica.  A hangar module concept also provides the opportunity to have it on star bases or death star.