"we" is a personal pronoun. "all" is an indefinite pronoun.
As you point out, "we all" is the subject, so two pronouns together can be the subject. As for the question of noun and pronoun, yes, these too can be a subject, for example, "We, the people, demand our rights!" "we, the people" is the subject.
The following may help you understand the statement that "all" can be singular or plural.
Singular use of "all": With non-count nouns, "all" goes with the singular form of the noun. As the subject of a sentence, it also goes with "is", "was", or any other singular form of a verb:
All of the meat was spoiled. All the butter has melted. Not all chocolate is sweet. Susa