For example what is the rule that says that, "Enough" (ɪˈnʌf) can be pronounced as /ənʌf/ But for "Introduce" (ɪntrəˈdjuːs) the /ɪ/ can't be pronounced as a /ə/?
Introduce actually has a secondary stress on the first syllable: /ˌɪntrəˈdjuːs/. Vowel reduction to schwa does not occur in syllables with any kind of stress.
Almost all English words have some kind of lexical stress (either primary or secondary) on either the first or the second syllable ("GIGO", John Wells’s phonetic blog). The second syllable of "introduce" can have a reduced vowel (as indicated by your transcription), which is a hint that the first syllable has secondary stress.
In fully unstressed syllables, reduction to schwa may occur based on complicated rules that differ between accents/dialects. Reduction to schwa does not always occur in all fully unstressed syllables.