Servo starts to shake when I added SIM900.begin(19200);
Servo.h use interrupts to generate the control signal. However, SoftwareSerial really messes interrupts up. 

You can't use SoftwareSerial with anything that relies on interrupts for timing, like Servo.h.

Solutions:


Use a better Arduino with an unused hardware UART
Use an external hardware PWM controller to drive the servos
Use the internal hardware PWM (you'd need to find a library to do this) to drive the servos.