My legions never have a completely identical organization structure and usually adapt to deal with whatever I'm facing.

In Gaul, for example, I may only use a small number of basic cavalry units since the need to flank isn't quite so great. In Greece, on the other hand, my forces will always have at least four (of the better) cavalry units to deal with the annoyance of phalanxes from behind. Likewise, if I see a few units of elephants, I will train some javelin throwers (Velites and Light Auxila) or if I'm fighting Parthia I will attempt to use a combination of long ranged and fast units, with heavily armored units to absorb enemy fire.

Essentially though, all stacks contain mainly infantry combined with a varying number of archers and cavalry.