Nuevo Leon state public security spokesman Jorge Domene Zambrano says four gunmen shot the first five drivers outside a taxi base office. Minutes later, three men were killed apparently by the same group a few blocks away.
Domene said two of the three people wounded in Tuesday's attacks were innocent bystanders, including the girl.
He said the victims were driving taxis without permits. Police are investigating a former driver who became enraged and threatened his co-workers when he was fired for selling drugs.
Also Tuesday, authorities in the western state of Michoacan said 13 men were killed.
Mexican soldiers shot four alleged criminals in an apparent kidnap rescue operation. And the state Attorney General's Office said three bound bodies were found on a highway, while six other bodies were scattered around the town of Tancitaro.
Prosecutors said the incidents were related. At both crime scenes, police found posters with messages signed by the New Generation drug gang, an ally of Mexico's most-wanted drug lord, Joaquin "El Chapo" Guzman.
In the resort city of Acapulco, police arrested three men who were the driver and passengers in an SUV with seven bodies, five of them decapitated.
Arturo Martinez, spokesman of Guerrero state, said Tuesday the three captured in the case are being investigated for ties with organized crime. He said some of the male victims had been mutilated.