In some languages, gender is distinguished in the plural. This naturally brings along the problem of what to do about mixed groups. It is fairly usual for mixed groups to default to a gender (all instances I know of default to masculine). What if a language behaved differently? What options are there? Here are a few with some hints at avenues of making the situation even more complex.
1) Free selection
The decision to use a feminine or masculine plural pronoun for mixed groups is left completely to the speaker.
2) Controlled semantic selection
Which pronoun is determined by some semantic fact about the utterance: maybe the gender indicates attitudes to the group, or maybe it correlates to TAM. Maybe actual, definite, pre-defined groups get feminine, whereas yet-undefined, hypothetical, future, potential groups get masculine.
3) Syntactic selection
- Subjects - feminine, other constituents: masculine.
- Main clauses: feminine, subclauses: masculine.
- Erg-abs part of grammar: feminine, nom-acc part of grammar: masculine.
- Certain verbs' or adjectives have congruence that is, for morphophonological reasons, defective: singular masculine and singular plural is conflated in the verb. In such cases, the feminine is used. In some other verbs, the opposite problem applies, and so the masculine is used.
4) Registral selection
The strategy may vary by register, but it may also be as easy as in formal registers, mixed groups are masculine, in other registers, they're feminine (or maybe free or whatever).
5) Speaker- or listener-based selection
Maybe mixed gender groups always get the opposite gender to the speaker (or the same), or maybe it's the listener's gender that determines. In case of mixed listeners, ...
6) Lexically controlled selection
Maybe some verbs prefer feminine pronouns for mixed groups, some verbs prefer masculine pronouns. The deficiency in congruence in 3.4 could easily be lexicalized and stop being specific to forms of the verb where the congruence fails.
7) Referent-affected selection
The group or some individual of the group, and some property of said person(s) affects the choice: majority female gets feminine, majority masculine gets masculine, or most socially prominent member determines gender of the group.
8) Feminine- and masculine mixed groups as separate referents
I am not sure this even could evolve, but imagine a system whereby e.g. masculine plural for a mixed group essentially is proximative, and feminine essentially is an obviative pronoun.
9) No mixing
Instead of a single pronoun, two are used: "they(fem) and he", "they(fem) and they(masc)".