In grammar lessons they say nunca triggers imperfect. Most of the examples in clozemaster seem to uses preterite with nunca. I understand the “nunca tuvo una respuesta”… one answer, never had. But here not having money seems like a continuing state
Those triggers don’t exist. They are just tricks to give the student time enough to apprehend the real inner working of the language (and trick in exams, as in the first courses they give some 70-80% of right answers without learning really anything).
Back to the examples, you have different use of the time frame and even different meaning of the words.
-Nunca tuvo dinero para pagar la renta —> He never paid it [I would like to find a single real instance of this sentence in the Spanishphere]
-Nunca tenía dinero para pagar la renta.—> Such situation repeated month after month.
-Manuel nunca fue muy inteligente. ----> a comment made after Manny did something clumsy. [Here the speaker is underrating Manny]
-Manuel nunca era muy inteligente.----> Manny’s choices were awful, and his lack of common sense made him take bad decisions once and again . [Here the speaker is underrating Manny’s decisions, maybe just decisions about some specific subject]