Ok, the DLC stories are completely separate it seems. I guess the items you have etc will be carried over to the second (?) half of the main story. The DLC stories are just side-stories but part of the same dilemma. But apparently you can import all your data from he separate stories in the final 800 point chunk when it all comes together.
It works like this-
Initial:
Prologue - Ceodore - Kain = 800 points
Tales:
Rydia, Yang, Palom, Edge, Porom, Edward, the Moon = 300 each
End:
Semi-final - Final = 800 points
Apparently there is a bonus dungeon/boss as well in the first 800point game. You talk to Namingway in Baron Inn.
Also, saves are SEPARATE so if you buy Rydia's chapter DO NOT OVERWRITE your original