There is not any specific or special requirements for generic medicine agency. You just need wholesale drug license to start wholesaling of medicines, no matter whether medicines are generic, branded or OTC.
For generic medicines wholesaling, you need to get distributorship or stockist ship of any generic company, in other words, we can say it agency. Or you can start by taking generic medicines from any other stockist but that will increase cost of products and will decrease your margin.
For taking stockist ship, you need to contact pharma companies those sell generic medicines.