Ultimately it's up to you as Legatus. Since you're the CO you have the final word on where your subordinates go, even if they want to go on a certain quest. Making sure you can handle the barbarians while some people go off to Greece to help Octavian get a few books is hard to work out with limited resources, so I'd recommend that you get some of our lovely Mercenaries to fill in the gaps created by multiple quests.

Of course, if you take Mercenaries that means the rewards will have to be split further to give them their pay.