Licensing servers with multiple roles

I everyone,

I´me having a hard time understanding how license works, for example:

- Will need 2x Agent for Applications and databases to suport for example Exchange and SQL Server on the same server? 

- Will need 2x Agent for Applications and databases to suport SQL Server 2005 and 2008 on the same server? 

- If I buy a Agent for Applications and databases for Sharepoint will it support the SQL Server Database installed locally (express or standard)? If true does the Database have to be used exclusevly for Sharepoint.

Thanks for your time

FranciscoRC