Installing instances of older versions of SQL Server Are there any issues to be aware of when installing an instance of previous version of SQL Server AFTER installation of a newer version? e.g. Installing 2008 to a server with an existing 2012 instance? 

I can find not statements specific to that circumstance, but it seems risky.
Installing an older version of the database engine should be fine and completely separate (though the superstitious side of me would prefer to do it the other way around). 

Don't install client tools or anything - keep it to the engine. If you need visual studio, BIDS, SSDT, SSIS etc., good luck...

Use the latest version of SQL Server Configuration Manager to manage the instances on the box, since newer versions are typically backwards compatible.