The order the Sins games were released are SotSE (or as we call it Original Sins), Entrenchment, Diplomacy, and Rebellion. If you want to play them in order then that is the order. Though like Gofan said there really is no need to, because every expansion has the content of the previous expansions in it. So you can play Rebellion, and not miss anything from the previous expansions. The only major changes to the game were from Original Sins to Entrenchment, and From Diplomacy to Rebellion.
Gal Civ doesnt fit in here at all (other than being a Stardock Game). He was just making an example of order of expansions.