Thank you very much for your reply! This helps a lot! 
I know that I will need an external supply for the servos and motors, I already run a few tests, but only with 2 servos. And even then I needed one.
I do have another question: I also found the MegaServo library on this site and it was mentioned that it was renamed to the Servo library. Do those two libraries have the same properties? In this library, it is also mentioned, that pins 44,45,46 won't have any PWM functionality when using the library. And it is mentioned which pins will loose this functionality when using more than 23 servos. Is this also the case for the Servo library?
Thanks a lot!