is it possible to control 8 motors with 74hc4051?
Yes, that will work fine. I would use 4.7K pull downs. Only one (at most) will be drawing current at a time. 

Put the pull-down at the 4051 output rather than the gate to avoid unnecessarily reducing the gate drive voltage.

Make sure your MOSFETs are logic-level drive (eg. 4.5V for a 5V MCU). With a couple hundred ohms in series with the gate they will not switch very quickly, so keep the PWM frequency reasonably low to avoid excessive switching losses.

The cross talk in the HC4051 will not be a problem. 

The diode is necessary, but the capacitor may be counterproductive.