I did a quick search on changing the PWM frequency.
It seems that this influences the timers that are used by commands like delay and the serial communication again.
So I guess it is not possible to change that (because erverything else stops working right?).
I also noticed that pins 4 and 13 on the board seem to run on much higher frequencys... so i cant use them.
The page about the mega2560 sais that it also provides pwm on pins 44-46.
But when i look on the pin mapping page of the 2560 it is very different 
It seems that there is much information on that boards that contradicts each other...
So my question is this:
- Is it possible to control 12 servos on the mega2560 without conflicting with timers, serial communication etc?
- which and how many pins do provide pwm output?
At the moment it looks like it is not possible to have 12 pwm outputs without changing behavior of serial communication or timers so maybe I just go back to my old hcs12 board and throw the mega out the window