Learn English – Is ‘hero’ applicable to females

The most common complements for the terms you mention are

• male/female
• boys/girls
• guys/gals

Usage is far from parallel. While arguably boys and girls should be reserved for underage (whatever that means) people of the respective genders, adults, especially younger adults (probably under 35) often refer to their own gender by the term of children of that sex.

However, refering to an adult using childish terms may be considered demeaning. It is generally more acceptable for men to refer to themselves and their agemates as boys and vice versa. When the child referents are used by the opposite sex, it is often seem as sarcastic or even insulting. Many adults will assiduously avoid referring to the oposite sex as a boy or a girl.

This is even more of an issue when dealing with groups that may have been forced into lower social status based on race or socioeconomic status. Do not refer to an African-American male as boy. This is highly insulting. To a lesser degree, an adult male referring to any other adult male, except for close friends as boy would be considered demeaning. The same rules generally apply to women (although possibly to a slightly lesser degree).

In some circles, even older women may refer to each other as girls (perhaps with a slight degree of irony), but such reference by a man may well invoce umbrage.

Finally, the use of the terms boys or girls for individuals over 70 may occur, but in an ironic fashion. Whether that is welcome depends on both the speaker and the audience.

Guy and gal are informal terms, much more polular in a period that ended in the early to mid 2oth century. They are still used colloquially, but often with a but of irony.

Many people use the term guys to collectively refer to a familiar group, regardless of gender. This term seems to be used somewhat more by men than women when refering to women or a mixed group of men and women. In general, most women do not seem to object to the term when used in this collective way.

The singular guy is generally applied only to men. To suggest a particular woman was a good guy would seem a bit off, regadless of who used it.

All ofthe above refers to US usage.

The overwhelming usage is applicable to. The google reports 67.5M hits for applicable to and 340K hits for applicable for. (Admittedly this not an exact gauge.) I find a similar disparity on the Ngram viewer. Applicable means relevant or appropriate, and when the subject under discussion applies directly to the person addressed, then applicable to is the phrasing of choice. Sometimes, however, the subject under discussion is relevant to the situation of the person addressed but not to person himself or herself. In that case, applicable for may be used. Here's an instance from Vicki Lansky's Divorce Book for Parents:

One rule of thumb is 25 percent of the noncustodial's take-home pay at the time of your divorce will go to child support. But only a professional can tell you whether this will be applicable for you.

So, generally speaking divorced parents who aren't living with their children (noncustodials) can expect to pay one-quarter of their salary to support their children. But both custodial and noncustodial parents may be reading the book, and the 25% rule isn't applicable directly to custodial parents, only to their noncustodial ex-spouses. So the author uses applicable for to include the situations of both types of divorced parents.