You'll learn some background information by playing the original, but not much. I think it's not wrong to start with an expansion, but I suggest you play the original one time, then perhaps the Dread Lords campaign, and then switch to the expansion packs. I don't recommend playing the original to death before trying the expansions, this will fatigue your appreciation for the gameplay a bit (allthough a game this good doesn't get boring that fast).

The Gal Civ 2 campaigns are good for following the story, if you enjoy that.  (I do).  The individual missions/maps for the campaign aren't terribly different than the sandbox game, but they tend to start small and limited and work their way up in size/complxity, so following them might be a good introduction to the game and the changes each expansion makes.

For the non-campaign sandox/skirmish game, I don't see a lot of point in playing anything besides TA - it's basically an enhanced version of DL/DA - it's got all the same stuff plus more, so unless you want to play without Twilight's additions, there's not much reason to go back, except for the campaigns.

So my recommendation:

- Play the campaigns in their orignal order: DL, DA, TA

- Move on to TA's skirmish/custom games and/or the metaverse.