Your first question is whether or not the words “all,” “some,” “both,” “few,” “many,” and “other” can only pair off with a countable noun and that they would require that noun as well as the verb to be plural in inflection.
This generalization applies only to “both,” “few,” “many,” and “other” when they are used not as indefinite pronouns but as adjectives, as in “Both girls resemble their mother” and as in these three of the four sentences that you provided as examples: “All men are mortal.” “Few students love Math.” “Many girls love Teleserye.” As adjectives, “all,” “few,” and “many” indeed generally can only pair off with a countable noun in plural form and require the verb to be in the plural form as well. A notable exception though is the noun “creation” in the sentence “All creation is the handiwork of God,” where “creation” is an uncountable noun that’s singular in sense, thus requiring the singular verb form “is.”
As to “some” used as an adjective, however, the countable noun and the verb can take either the plural or singular form. In this particular sentence that you provided, “Some clothes are already old,” the countable noun “clothes” is obviously plural, thus requiring the plural verb form “are.” But when the adjective “some” is used as a modifier in the sense of being an unknown, undetermined, or unspecified unit or thing, the countable noun it modifies can also take the singular form, as in “Some guy is courting Mario’s sister” and “Some part of the engine is malfunctioning.” The usage of “some” here indicates that the writer or speaker is uncertain of the identity of who or what is being talked about.
It is when they are used as stand-alone subjects in a sentence that “all,” “some,” “both,” “few,” “many,” and “other” function as indefinite pronouns. As such, all of them are plural in sense and require the plural form of the verb, as in “Both resemble their mother,” “All are mortal.” “Few love Math,” and “Many love Teleserye.”
Now to your second question: When working as indefinite pronouns, is it possible for “some,” “all,” “none,” “any,” and “most” (“SANAM” for short) to be followed by an “of”-phrase with a countable noun in singular form, as in “some of the student”? The answer is absolutely no. When any of the SANAM is used, the countable noun that follows the “of”-phrase should always be in the plural form, as in “Some of the students are absent,” “All of the players are excited,” and “None of my friends is coming today.” That’s just the way it is with the indefinite pronouns in English.
