*disclaimer:*I'am still learning hiragana, so forgive me for any wrong spelling
I've learnt that normally only males use ぼく (boku). What about ぼくたち (bokutachi)? Should it normally be reserved for male-only groups or can it be used by a male to refer to a mixed-gender group of "we"? Can a female use it to refer to a mixed-gender group?
〜たち is a suffix which means more or less "the group to which <someone> belongs". So ぼくたち refers to the speaker as ぼく, but it also refers to the group to which they belong, and it can be used as long as ぼく is appropriate for that one person.
In other words, it's not a "plural" marker, so it doesn't matter whether the group is mixed-gender or not. They're not all being identified as ぼく. Only one person is.
Of course, the same thing is true if you attach 〜たち to a name. If I write 田中{たなか}さんたち, I'm not referring to a bunch of 田中さんs. I'm referring to one person as 田中さん, and I'm also referring to the group to which 田中さん belongs.
And finally, the other similar suffixes in Japanese, such as 〜ら and 〜がた, function the same way. 彼{かれ}ら refers to 彼, but also to the group to which that person belongs. So it can be used for mixed-gender groups too, as long as it's okay referring to the person in question as 彼. And so on.
It is perfectly normal for ぼくたち to refer to mixed-gender groups. For example, when I talk to a third person about something my wife and I are going to do together, I say ぼくたち. Generally, when referring to groups of single or mixed gender of which they are a member, males use ぼくたち (or some other masculine variant like おれたち) and females use わたしたち (or another feminine variant like あたしたち). However, this is not a hard-and-fast rule and there are many, many exceptions (my favorite example being the female character 九兵衛【きゅうべえ】 from the manga/anime 銀魂【ぎんたま】 who always refers to herself as ぼく and therefore also frequently uses ぼくたち).
