No expert sphere grid option that I know of. American version.. not international

And just to clarify... having a overpowered Tidus/Lulu/Yuna/Auron is bad? or just that some fights require abilities of Kimari and Wakka, and therefore having them weaker is bad?

Wakka has leveled decently.. just hasnt seen as much as the other four.
The ronso however, hasn't done but maybe 7 spheres or so.

If I had to guess.

Tidus: 55 spheres
Auron: 45 spheres
Wakka: 25-30 spheres
Lulu: 50-60 spheres
Kimarhi: 5-10 spheres
Yuna: 50-60 spheres

Spheres may be wrong count, but general %s you can derive from that.
I know there are times where Wakka's hit % will be needed, but wasn't sure about the Ronso.

And being overleveled is bad?
Besides the challenge factor aspect.

and i'm confused.... I'm not relying on any single character....

I'm relying on a constant party of Auron/Tidus/Yuna.. with Lulu switching out for Tidus at times, and Wakka for anyone if needed.