AFAIK, you can not apply that in Spanish, You should use the masculine plural gender to refer to both genders, or when the gender of the people is unknown.
There are exceptions to the rule, like the one above (Zapatistas) or some professions like "doctores" or "jueces", but they are exception not the rule. There are also many words that dont have a feminine plural, i.e "las soldados", "las jueces", etc
In Italian it is true that the last letter is not a perfect indication of gender... even for people's names (e.g. Andrea should be a male name). But it works pretty well. Not sure if Spanish is similar (i.e. there are exceptions) or not.
There are exceptions to the rule, like the one above (Zapatistas) or some professions like "doctores" or "jueces", but they are exception not the rule. There are also many words that dont have a feminine plural, i.e "las soldados", "las jueces", etc