>With one SSMS (no matter where it is installed) you can connect to every instance of SQL Server (if you have proper authentication of course).
To be on the safe side, it's better to install the latest version of SSMS (SQL 2014). I am not 100% sure you will be able to connect to higher version of the Server using lower version of SSMS.
If it's not broken, fix it until it is.
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