For example, if I choose the original leaders from Native in the Expansion, will I get the same gameplay rules, maps, units and variables as if I were playing in the original game? i.e., Does the expansion change the gameplay drastically?

I want to know if I could play both Expansion and non-Expansion games on Alien Crossfire, or if I need to use both programs to play on each game's exclusive rules.

You’ll still have access to the new techs and buildings of Alien Crossfire. No. A bit more variety, but no drastic gameplay change. You can play both on Alien Crossfire. Playing Alpha Centauri vanilla is required only if you don’t like some of the things added by Alien Crossfire (techs, buildings and unit parts, I might be forgetting a couple things).

The available races are the main difference,

In the original game, Gaians are way overpowered and Morganites are way underpowered so i would use them as part of my adjustments to the difficulty level i wanted to play.

I think the races in the expansion are a bit better balanced, possibly a bit too well balanced making the game feel somewhat less interesting somehow - even despite the two alien races?

I consider the expansion unbalanced. It's factions are weak except for the overpowered aliens and the secret projects it introduces are overly powerful.

I wish!

well, overpowered against some of the other weaker AIs.

True, but i would bet those alien races would not look overpowered if they spawned near the Believers from the original game!

Anyway, i always check my games at the start with the editor and see that all the races get a good starting point so they don't wipe each other out to quickly. Sometimes i might even have to move the races around, but then i cannot reload my saved game before opening the editor so that the score will remain (not that i pay attention to scoring).

Hot Tip: weaker races benefit greatly from starting on a monsoon jungle!

I think everyone benefits greatly from starting on that jungle!

I'd SERIOUSLY query the Gaians being the strongest faction. From the way the computer plays I'd rank them 1) Believers 2) The Hive 3) The Spartans 4) University 5) Peacekeepers 6) Gaians 7) Morgan.

I'd also challenge that the new discoveries are overpowered. There's nothing as game changing as the Hunter Killer Algorithm, The Cloning Vats, or the Telepathic Matrix in the new discoveries.

The aliens are MASSIVELY tough compared to humans. However they are doomed to fight each other come what may and humans are more likely to unite against them than with them due to energy bonuses. That is a balancing factor.

The main differences are - 1) It takes longer to get to the end of the game because there are so many more discoveries 2) For most of the game tougher defences arrive earlier making it more difficult to land early knock out blows. 3) The String Gun (at 30 attack) gives a chance against the toughest defences. 4) You can have an entirely random game for starting factions which is cool - even including your own faction.

S.x.

You could use the Gaians to grab some early victories using mindworms but in the long run I don't know that they're more powerful than any other faction. I'm not aware of any tech additions from the expansion, only secret projects.

Here goes the list of the new techs added by Alien Crossfire:
Progenitor Psych (E1)
Field Modulation (C2)
Adaptive Doctrine (C3)
Adaptive Economics (B3)
Bioadaptive Resonance (C4)
N-Space Compression (C8)
Sentient Resonance (C9)
Secrets of the Manifolds (D13)
String Resonance (C16)

The expansion gives you some nice little bonuses to harvesting energy and nutrients from the oceans; also there's attachments like Soporific Pods to add to your units. I like the expansion, it's really fun to play on a map of planet with the original factions and the extra options. It's also cool to mix and match factions.