Okay, I'm looking at Nock's edition of the Greek (and his translation) as I write this comment.
The first sentence has "the Gods" in the plural. That means exactly what we'd expect.
Later, in the bit about "common conceptions" (which Nock translates, perhaps more helpfully, as "universal opinions," in other words, beliefs shared by everyone), the Greek uses a grammatically singular construction, hoti pas theos agathos, hoti apathēs, hoti ametablētos. The key term here is pas. Taylor (as quoted in the post) renders pas as "all." Really, this term is a universal quantifier, so while "all" is fine, a more felicitous translation could be "every" or "any" in the sense of "any imaginable, any possible."
So the entire phrase means: "that every God is good, and impassive, and unchangeable." That preserves the grammatical singular of the Greek; we could just as well convey the same idea using the plural in English: "that all Gods are good, and impassive, and unchangeable."
Even in other contexts where the word "all/every" is not given, Sallustius and his fellow pagan Platonists would only use the singular form theos in one of two ways: (1) in a context where an individual God (perhaps Zeus or Dionysos) was already referred to by name, and they were just mentioning that same God again without repeating his name, or (2) as a generic singular to refer to the class, in just the way that someone might say "Man is the measure of all things" or "man is a rational animal," thereby referring to human beings universally or in general, without in any way implying that there's only one.
(In Greek, unlike in English, you use the definite article—"the" (ho)—in both those constructions (ho theos, ho anthrōpos) to refer to the class or the generic. It's a usage that, because of Christianity, is almost impossible to convey any more in English. Sigh.)
Re: perfect, therefore unchanging
The first sentence has "the Gods" in the plural. That means exactly what we'd expect.
Later, in the bit about "common conceptions" (which Nock translates, perhaps more helpfully, as "universal opinions," in other words, beliefs shared by everyone), the Greek uses a grammatically singular construction, hoti pas theos agathos, hoti apathēs, hoti ametablētos. The key term here is pas. Taylor (as quoted in the post) renders pas as "all." Really, this term is a universal quantifier, so while "all" is fine, a more felicitous translation could be "every" or "any" in the sense of "any imaginable, any possible."
So the entire phrase means: "that every God is good, and impassive, and unchangeable." That preserves the grammatical singular of the Greek; we could just as well convey the same idea using the plural in English: "that all Gods are good, and impassive, and unchangeable."
Even in other contexts where the word "all/every" is not given, Sallustius and his fellow pagan Platonists would only use the singular form theos in one of two ways: (1) in a context where an individual God (perhaps Zeus or Dionysos) was already referred to by name, and they were just mentioning that same God again without repeating his name, or (2) as a generic singular to refer to the class, in just the way that someone might say "Man is the measure of all things" or "man is a rational animal," thereby referring to human beings universally or in general, without in any way implying that there's only one.
(In Greek, unlike in English, you use the definite article—"the" (ho)—in both those constructions (ho theos, ho anthrōpos) to refer to the class or the generic. It's a usage that, because of Christianity, is almost impossible to convey any more in English. Sigh.)