Do I want to add the expansion into game before playing the first game?
I know for some games you really want the expansion early beacuse it balances things that were slightly out of wack in the orignal or it adds a fun mechanic that makes the game more enjoyable. Other expansions, however, are designed to refresh a game that people have played so many times they are starting to get tired of it or to shake up old strategies.
Which is the GSF expansion?
I felt I had plenty to keep track of in my first game and didn't need anything new or expansion-ey complicating things. So my opinion would be to keep things simple and leave the expansion in the box until you've gotten a few plays under your belt.
The expansion really doesn't add any new layers, but it certainly makes 2 player games easier just because of the station card Reclaimator alone (flips into a ship that can engage SF ships or other Opps. Very useful).
The only thing the expansion adds is some station cards that will provide you with artifacts, which are a 3rd resource that some other expansion station cards can use. So if you don't buy them, nothing changes, but they are very powerful cards and well worth using.
I just added in the expansion with everything else when I unboxed everything. In our first (and so far, only) game, I got hold of two copies of a Boost card which let me gain a pile of artifacts, and later ended up with two copies of a Tech card which let me lose artifacts instead of energy at any point that I would lose energy. We were doing damage calculations a bit wrongly (removing energy directly instead of first deducting soak granted by shields) but even so, I didn't lose any energy in the one or two times I got hit because I just removed the artifacts instead :D.