Yep, they chose the most important tribes of each generalized faction in vanilla, and did their best to portray the lesser tribes as best with the rebels. It really boils down to 1) the limits of the RTW game, and 2) the sheer complexity of the historical situation.

The units thing is tricky at first, but you will catch on soon enough. Try the Eb website, www.europabarbarorum.com, for background information on the factions, their history, and their units.

As for the Romans vs. Gaul thing, I'm no expert or even enthusiast on the matter (I'm more of a diadochi guy), but I'm pretty sure that the Romans fought the Gauls on different occasions at different times (so many Celtic tribes) but in Ceaser's Gallic wars, I'm quite sure that the Gauls were pretty much unified under Vercingetroix, though for the details of this topic, hopefully someone else will chime in...