As for "self-perfect essences", I think the way Neoplatonism thinks of gods is that they're causal fixed-points: they're those things that act but are never acted upon. Therefore the various gods are all the things that can act in such a way that they are self-stable (e.g. they don't blow up or burn out, since then they wouldn't be eternal). Thus they're "perfect" or "complete" since they each have all they need to sustain themselves indefinitely.

(I assume there's one of these for each way in which it is possible to act in a self-stable manner. Personally I would have guessed there's an infinity of ways for that to occur? I suppose the Greeks figured that there were exactly twelve, or maybe I'm reading into their myths wrong. We're not at the chapters on myth, yet, anyway!)

Speaking of "perfect", Neoplatonism talks a lot about "the good," and last week I wondered what the deal was with that, since they clearly mean something other than "beneficial?" Maybe "perfect" is a better translation than "good" in this context (since it has both connotations of being "beneficial" but also "complete", and it feels like that last is being relied on to make all these little proofs that Sallustius is making).