How can I use 15 servos with arduino mega2560 without stepping down voltage?
I think you haven't worked with servo motors yet. They requires external power to drive the motor(as they require high current which cannot be delivered by microcontoller) 

Servo motors will have 3 pins 

1. +V(recommended voltage for servomotor)

2.Controlsignal(pin from microcontroller output) 

3.gnd(common gnd).


Hence there will be no point of stepping down or up the power..