The Mac version of Reign of Giants seems to come as just a complete game with DLC already installed. If you want to play regular non Reign of Giants do I need to install the old non DLC version of Don't Starve? Or can I just turn off the DLC from inside.

In general, when you install Reign of Giants, it adds a button on new game screen that lets you toggle it on/off, so you can chose wether to play RoG or not on a per-save basis. It also pops up the window warning you if its your first run that RoG is harder and you may want to disable it.