Balance? Frankly, I'd just rather them respect the other factions rather than make them stronger. Give the Gauls there unique units, give the Germans theirs, and the Celts theres. Don't smush them all together. Frankly, I don't care to much about balancing, and would rather have the game that respects all the factions and has historical accuracies than a game that has all the factions equal (since, as you pointed out, they weren't).