Different fee structure for different clients? A violation?

A and B are both clients of my firm, under the same investment strategy

If I charge Mr. A a management fee of 2% and 20% of profits.

And,

I charge Mr. B a management fee of 2% and 20% of profits in excess of the S&P 500 return,

it can clearly be seen that the two fee structures are different.

Am I in violation of the Code of Ethics / Standards?

No if both fees are on reasonable basis and are transparent and in agreement with clients. A firm must treat all clients fairly. Fairly does not mean equally.