I don't know what exactly you read, but while Rebellion has all the content of all the Sins games, you can not play the previous versions as they were. You just have the Stand alone package of Rebellion with everything.

Really though, unless you have friends that only have older version or you really want to play some now outdated mod, there is little reason to play the previous games. Just look up the Original Sins and diplomacy introduction videos on youtube and you'll get all the story.

I'm guessing you did not do much research of Sins before you got it (or the reviews just mentioned what Rebellion adds, not what Sins is). There is no guided campaign or story, in any of the games. It is based more on the "Make the story as you play" style of older 4X games like Master of Orion II. I have not played GalCiv but if you're expecting a full campaign like Starcraft or something there just is not one.

While I realize a lot of people like campaigns so they can learn the game, Sins is a really quite unique for an RTS; few others have its depth or scale. If you need help learning, try looking up some stuff in the strategy forums here, or look at some youtube videos or guides.

I don't think there was ever a campaign, though there are many options for customization that allow you to have games of any size. If you use Galaxy Forge (which will have come with your Sins files, should be in the documents) you can create your own maps to your exact specifications, save them, and play them.

If you're a noob, I like to get new players together for games. I have a bunch of friends who play Sins but the competitive crowd scares them away. I do custom maps and stuff, as well.

If you'd like, you can add me on Steam and we'll play.