hi, I vaguely recall playing this game long ago with alien crossfire but being able to play all factions under the expansion. on the gog version it is only the expansion factions that are available. am i misremembering or am i doing something wrong with the gog install?

While setting up a game there's three windows. Left is OG factions, Middle is Expansion, and Right is Chaos Mode. (Selections made by you.)

Note however that you cannot play with all factions as there's literally not enough slots.

And you're running TerranX, right?