I have been playing with a legion being:

1 general
1 equites romani
2 triarii
3 principes
3 hastati
3 balearic slingers or 3 cretan archers
2 scutari or 2 peltastai
and then whatever mercs I deem neccessary for the job, usually cavalry.

The exact composition depends on where the legion is based. I have one in Emporion, one in Lepki, one in Segestica and one in Demetrias. The western legions have balearic slingers and scutari and the eastern legions have cretan archers and peltastai.