I made my default team Lightning, Fang and Vanille. I didn't like to switch the party around much because they made reassigning your paradigms such busy work. I will admit that Lightning, Fang and Hope is a better default balanced party, but I couldn't stand Hope, so I made due without a dedicated Synergist. I tried going with Lightning, Fang and Sahz for a while when I first arrived on Gran Pulse, just making Lightning my healer, but against some of the game's stronger monsters, she just can't cut it as your primary healer. So I traded Sahz for Vanille and made due without a dedicated Synergist. And it worked out for me in the end. I minored all three of them as Synergists, and it worked out pretty well. Fang gets only a basic set, but it includes Haste, which is probably the most important Synergist spell. Lightning gets almost everything but the Bar- spells, so she makes a good Synergist in then end if you're willing to put the time in. Master's Seal Achievement/Trophy requires that anyway. And Vanille is a better Medic than Hope, since she has much higher health and will survive longer to keep supporting your efforts to make enemies dead.

In the end, with the right equipment, you can probably make any party work. Some just take a bit more grinding than others.