This is not exactly accurate. If a company pays its CEO with stock instead of salary, the value of the stock is still taxed at the normal income rate. If that stock later appreciates in value, then that increase is taxed at the capital gains rate. In other words, there is no difference between a CEO receiving $1,000,000 in stock as compensation, versus a CEO receiving a check for $1,000,000 and buying the stock on his own.
Regarding social security tax, there would be no reason for him to keep a low salary if your theory was true. You stop paying social security taxes beyond your first $106,800 of income anyway, so it doesn't really matter if he earns more than that.
Regarding medicare tax, it's roughly the same story as the federal income tax. Your taxable income for medicare is just your AGI from your income tax, minus certain deductions and exemptions. You still have to pay medicare tax on stock compensation that you receive in exchange for your labor.
Dividends are paid out to shareholders; it is impossible "take income in the form of dividends." He receives stock (which is taxed at the normal income rate), and then that stock produces dividends depending on the company's performance (which is taxed as income just like for every other shareholder).
This is a very odd criticism, because it is generally considered a GOOD thing when CEOs take their compensation in the form of stock rather than salary (because it shows they are more invested in the success of the company). Furthermore, the information Buffett released explicitly contradicts your claims. His taxable income was nearly $7 million, so clearly it includes more than just his $100,000 salary.
At first glance there doesn't appear to be anything unusual in the information he released. Still, I would agree that it was probably not a good idea for him to release it simply because any wealthy person is going to have some stuff that his critics will attack him for. Which is exactly why the Republicans wanted him to release his tax information in the first place; not because they were actually interested in examining the problem with the tax code, but because they wanted to nickel-and-dime his deductions in order to use it to discredit him.