Cities cannot be placed next to each other. It is unclear to me if this rule also applies to the Noctis city hex before Noctis city has been placed? Personally it feels wrong to put a city next to Noctis, but I have seen others doing this so wondered if this had been clarified officially?
No official clarification needed. If Noctis City is not built you are breaking no rules building a city next to it's future position.
As for Noctis City itself, its card is explicit that you get to ignore certain building restrictions (ie building next to other buildings) when building Noctis City so everything is fine.