Installing instances of older versions of SQL Server
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.