I tended to go with Lightning, Sazh and Fang because Sazh gets Haste before Hope does. I was doing Lightning Hope and Fang before then, but with Haste things become a lot easier. Having Hope around is good because like Vanille he is the only other character with Medic access to the higher level healing spells. But for most of the game you won't need them. Also you get exp for everyone so in the end you'll never be underlevel if you feel that you made a mistake using one character over another.

And equipment upgrades are pretty much unnecessary in the long run for completing the game. Doing the post-game work certainly, but you can beat the game without being too worried about how well your equipment is leveled up. In the end you'll need to level it all up if you're going for the trophies.

What they is certainly good advice to have access to all of the roles in your party, but work with what suits your play style. Because you can beat the game with any of the characters people have done it. It is the nice thing about the game, there is no really wrong answer for your party. Only what does and doesn't work out for you.