So, after watching Across the Sea (6x15), which gave us a lot of answers about what is the island, who is Jacob and who are Adam and Eve, although not the way many people were expecting, I realised that we got two, insignificant though, answers about candidates and requirements to be one of them. On with the article:
First of all, when the candidate thing was first explored, on The Substitute (6x04), many people were asking themselves if Kate, as a female, was a candidate  - and if Jin and Sun were both candidates, and if not, who was the one .
1 - We already expected that females could be candidates, as names such as Burke and Rosseau were part of female characters only. But now we got the confirmation. Watching this episode we could conclude that before Jacob being in charge, his adoptive mother was the one in control protecting the island. We can then assume she had the same status as Jacob, meaning that females can be protectors of the island.
2 - The Jin and Sun stuff was also answered as when The Source is revealed, Jacob's adoptive mother tells both Jacob and MiB that only one of them can be the protector of it. Assuming that candidates are recruits to Jacob's position, this means that both Jin and Sun are candidates, but only one of them would be the protector, maybe being the one accepting the role or being the last one to survive, which unfortunately didn't happen. This means, after all, that there is no mistery surrounding wether the candidate is Jin or Sun.