The International Telecommunication Union allocates orbital slots, generally for Geosync. So yes, countries do have to reserve slots. Specifically, the Radiocommunication Sector (ITU-R) division of the ITU is responsible for orbital slot allocations.
A broadcast or communications satellite needs to be properly spaced to avoid frequency interference/overlap. But a weather satellite, or earth observation satellite might not interfere and could coexist.
There is a ton of great detail on the Wikipedia page for Geosync orbit.