If you expand into Central/Northern Europe, you may wish to hire Gaeroas or Dugundiz as mobile spear units, to coer the flanks against cavalry as well as to act in all kinds of supporting roles. Dugundiz are more reliable, while Gaeroas have more missiles and a wider AoR. I like to hire these because I feel that Romans (especially after their various reforms) are a bit short on spear units, and spamming Triarii is both unhistorical and relatively inefficient (restricted AoR, and high vulnerability to the likes of Sloxonez or Drapanai - against whom the aforementioned Barbarian spearmen carry javelins). Also, the Alps provide all sorts of versatile and reliable regional infantry.
When fighting in woods, logic dictates caution, i.e. not spreading/fanning out too far in any direction. Flexibility is key, and you should use flexible infantry units. I.e. those with more than one weapon.