Group composition principles

The short version is - are there any principles or rules of thumbs about comprising your group? Ie. go with a healer, martial, ranged and support character for your group composition?

I’m running lazeil and karlac as pure fighter and barb respectively, with my tav (sorlock), shadowheart (pure cleric), and it’s been a lot of fun, but I’m currently in the middle of the gith creche part of act 1 and having a hard time with some fights. That’s all made we wonder if the karlac and lazeil martial focus is a bit much and if I should take a different approach to compiling my group.