I used Solas as spirit/ice, since ice has less interesting actives. I made Viv pure lightning and Dorian pure fire; right down to the staff. I also give them each barrier and aggro reduction passives. I give them all fade step and the mana regen passive in ice. Plus their specs, of course.
This PT I flipped Viv and Dorian, partly because I find fire the least interesting of the three, and Necro is also least interesting spec of the three, boring combo. Also I wanted to throw static cage with one, and KE with the other.
Built this way, I mix and match depending on the region and the dragon. Ever since FFXII, I've become weary of making everyone the same/generic. That is what I liked about FFX, where everyone was really different, but could steal from another character certain threats depending on where you were on the sphere grid. Khimari in the middle could do whichever you wanted to borrow from.