First things first: What are the ancestors? They are beings like Celestia and Luna-controlling their respective element and being the first of the world. Many of the the Ancestors once lived in what now is Sulvania, until their war with dark. The Mother of all Ancestors, Creation(otherwise known as Lauren Faust to Princess Celestia and Luna) created the Ancestors to bring balance to the world she sustained in the midst of the War between Good and Evil. More information of the Ancestors will uncover as history unfolds.
Now, to the other part: there is not selective order these stories are in(except the Roleplay ones). Now, the roleplay ones are in story format, but they won't be written in the story because they take place after. Anyways, the Roleplays are interesting, so do your best to keep up!
*Marked Random because it involves many different categories*