is it ok for primary keys to be the same across tables (i.e. can the primary keys of "Clients" be 1-2-3-4... and the primary keys of "Staff" to also be 1-2-3-4...)?
or do they have to be completely unique throughout the database (i.e. "Staff" has s1-s2-s3-s4... and "Clients" has c1-c2-c3-c4...)?
They can be the same. I would suggest using a PK auto-numbering scheme; but also a second "soft key" that would have something a little more readable, like a userid that you can identify easily in a query as your database grows.