SDAC does not work with SQL Server directly. SDAC can work with SQL Server using two providers: OLE DB that is shipped with MDAC, and SQL Native Client that is shipped with SQL Server. You can find the differences between these providers at
http://msdn.microsoft.com/en-us/library/ms131035.aspx . As MDAC is supplied with Windows, you are able to work with SQL Server without SQL Native Client.
On the other hand, native client is also needed for Interbase/Firebird?
Yes ,to work with InterBase/Firebird, you should use theirs client libraries, gds32.dll and fbclient.dll correspondingly.
And I want to know that to access MS Access, Sybase, DB2, DBF is only by ODBC?
Yes, you can work with MS Access, Sybase, DB2, DBF via ODBC only.