Are there other human settlements in Gran Pulse

You are hugely underpowered for the majority of the enemies on Pulse, since Pulse is the end game location - your characters have access to that area and all the missions after the main story of the game is completed.

At this point, you should just finish the missions you need to move the story forward and don't worry about any of the enemies that are too difficult to defeat. Once you are done the story, you will be able to defeat most of the enemies in Pulse and you can spend time grinding in order to defeat them all (and finish all the missions).

Gran Pulse is littered with enemies of all strengths. Some are so powerful their just walking about will destroy you. For the most part, powerful enemies are reasonably easy to avoid. Such enemies won't be defeatable until you are well into the post-game.

As for the Cie'th stones, you don't have to do them all (only certain ones are required to advance the plot). I would recommend doing 1-14 or so to get some important unlocks (Chocobos!), and following the Cie'th stones will generally lead you down the story.

You can quit a mission without penalty and come back to any area in Gran Pulse to get the treasure again, especially in the post-game. Though on Cocoon there are many areas you can't visit again after leaving, so you want to be more thorough there during the "tutorial".