They've actually been doing a good job balancing out classes to the point where any sane collection of party members should be enough to get you through to the story credits, at least. So don't feel too stressed-out about needing to pick everybody exactly right on the first try.
I like to have a few different situations in mind:
- What am I going to do for healing?
- What am I going to do for physical damage?
- What am I going to do for elemental damage?
- What's my strategy against encounters with lots of enemies?
- What's my strategy against single meaty targets like bosses?
For this game, I'm going with an Arcanist and Sovereign to cover most of my healing, along with Arcanist's circles to cover encounters with lots of enemies. Sovereign's buffs can provide a source of elemental damage. I've got an Imperial to go after meaty boss targets with elemental damage (eventually, once the elemental drives unlock). I've got a Highlander to provide physical damage both for wide-area and single targets. And I've rounded things out with a Pugilist who can shut down enemy attacks in lieu of having a tank type of some sort.
This setup might need to be shuffled around long-term or I may have to rest some characters in order to reassign their skills better, but that's all part of the fun. Go with what seems interesting and don't be afraid to experiment a little along the way.