Thanks. So, what I have well understood is: do not try to move servo motors in manual way! otherwise gears could be definitely damaged. Ok, clear 
I do not understand the reason why 4xSG90 plus 1xMG90 servos could be rotated manually but another MG90 not...maybe is a matter of tolerances among each gear or some other reason which I do not understand.
One question, why should I not use arduino to supply servo? It’s a typical layout which I found around. The servo gets supply from 5V pin. Its ground is connected to same GND of arduino. I use a potentiometer and a capacitor....same layout found on Arduino basics...what’s wrong?