You can even fry it with 0V outputs. 20V is outside of the admissible maximum ratings. There are ratings for voltage, current temperature and some more subtle ones. Driving FETs is often considered uncritical however you have to be somewhat careful, especially if you drive lots of high powered FETs at high frequency. Basically they are a capacitive load and thus it is possible to exceed the maximum current rating. If you add some resistor between the Arduino and the gates (e.g. 1k) your Arduino will be fine. If you frequency is high enough your FETs might then suffer though.