(1) No problem to open a lower version of SQL Server from a higher version SSM such as opening the SQL Server 2005 Database Engine from SSMS 2012.
(2) SSMS 2008 and SSMS 2008R2 uses the same SSMS interface.
(3) When you open a higher version database engine from a lower version SSMS such as opening SQL Server 2012 database engine from SSMS 2005, you may receive an error message as below:
Index was outside the bounds of the array. (Microsoft.SqlServer.Smo).
Per Microsoft, this error will not be fixed. Instead, it is recommended to open the database engine with its compatible client tool - SSMS. For more information on this issue, see:
https://connect.microsoft.com/SQLServer/feedback/details/666386/index-was-outside-the-bounds-of-the-array-microsoft-sqlserver-smo-while-connecting-from-ssms-2005-2008
