Hi, I missed these games when they came out so I am wondering how to play. Do I play Alpha Centauri first and then Alien Crossfire or is it more of a normal 4X game where I should skip to the one with the most options. I've heard that there is a "plot" in this game, which is unusual for a 4X. Would that impact my choices or am I supposed to "know" certain things if I start playing Alien Crossfire?

You can play Alien Crossfire (SMAX) without having first played the original game (SMAC). The SMAX back story adds to the original. It doesn't replace it. Some believe that the seven original factions are better balanced than the new ones in SMAX. You could start a custom game of SMAX and choose to play the original factions. That might make it easier to get started.

I think Petek got it right. I play SMAX with the original seven factions. The human side of the story is the same as the original but the expansion adds a few new technologies, a few new kinds of native life forms and some game balance tweaks among other things.

I think I prefered SMAC alone where the alien artifacts were just a mystery. SMAX exists pretty much solely to explain those mysteries and make the aliens playable (in addition to tweaking and adding more gameplay content). I'm sure many liked to discover more about the grand design purpose behind the 'Planet', but I thought the setting was good enough when it was just humans trying to assert themselves and their ideologies on a new frontier.

I think SMAX is a very impressive expansion. It is not a new campaign or a spin-off, it is additional story elements and game contents added on to the original. It is kind of like a director's cut that includes story arcs omitted in the theatrical release, though this is not a perfect analogy because there are some differences in key personel developing the original game and the expansion. Perhaps it is not exactly how the original designers had intended the SMAC universe to be. No matter, because SMAX is an official game and so it is also true canon.