Developing Personas

General question about developing personas. I work for an organization that provides 6 core services to customers/users. I want to introduce UX research practices to my organization, and want to start by working with our internal departments to develop a set of user personas. Using these personas I want to work on improving our end-to-end customer/user experience through a combination of UX research and design techniques.

Being new to the UX research field, I have a general question about developing personas. Should the personas be made at the highest level? Ie. Should they represent all of our users, across all services? Or should the personas be unique to each service. (Note: We know for fact that the majority(~95%) of users use 3 of our services.)