"They are all girls."
Is みんな the only way to say "all" in this context? 全ての人 seems to be another common way of saying "all men/people", but I'm not sure if can be used here, I suspect it's more like "every man"/"every woman". In fact "all" is actually more like an adverb here, meaning "without exception" or "entirely". What about 全員?
For Subete no hito(tachi) i'd use it when I'm saying "All of humanity". Unless you say subete no minasan or something in which the minasan is already addressing a defined group of people.
Zenin means everyone, but it also talks about a defined group of people, as in the company employees, family members etc