Custom Line Manager Role for MSS Functions

Today, we use custom Employee Role for limited ESS usage, and a custom Line Manager role for almost no MSS usage. Our HR Manager wants us to expand our use of ESS/MSS functions.

We are just beginning our analysis in this area, and have some questions:

My Manager wants to know if we should start with the seeded Employee and Line Manager roles, stripping out what we don't want...or if we should start with our existing custom roles, and attempt to add missing privileges to the roles.

Also, there are a number of privileges in the seeded Line Manager role that we would not be using, but would not provide unwanted access to the application (eg: REST Access roles). Should we strip these privileges out of the role, or leave in as much as possible to avoid making unnecessary changes to the role?