so now i have to enable pull down for all 16 additional gpios by adding resistor between pin and ground,right?
Yes if you want the period between the power up or reset, to the code running to have the value of zero. This is sometimes needed if those pins are eventually going to be outputs and you have wired the outputs to be active high ( a common rookie mistake ). Then you might need a pull down resistor on those pins but more commonly you would design the system so that peripherals are active low.