About Final Fantasy Xiii Cie'th Stone missions?

Hi,
You can do them in any order but only some are available at one time. As you complete more, others become lit on the map as being active. For the most part they do increase in difficulty as the number gets higher. If you are taking them on when you first get to Gran Pulse, doing them in order is probably the way to go. You could probably tackle about 20 of them. I came back after beating the game and did them in random order but some of them are still really hard. Hope that helps.