On this site as discussed by me and Kirin yesterday the ranks don't have limits on the level of jutsu they can use....so yes genin can use S-ranks...however if you dare as a genin you are actually finished thats how much it would suck the chakra out of you, it would seriously be one shot its hit or die really....
Amounts
Genin (C-rank):
D-rank: unlimited
C-rank: 5
B-rank: 1
A-rank: None must be purchased
S-rank: None must be purchased
Chuunin(B-rank):
D-rank: unlimited
C-rank: unlimited
B-rank: 5
A-rank: 1
S-rank: Must be purchased
Jounin/anbu(A-rank):
D-rank: unlimited
C-rank: unlimited
B-rank: 10
A-rank: 5
S-rank: 1
Kage(S-rank):
D-rank: unlimited
C-rank: unlimited
B-rank: 20
A-rank: 10
S-rank: 3
Sat Jul 23, 2011 1:51 pm by 
