I believe it would be advantageous to select outposts on these single-slot moons for the desired system stats, not because they have a refinery.
It depends what you're after.
If you're after total highest refined goods output from the system (which was the question I was thinking of there), then adding a T1 outpost (which the planet flips to refinery economy) on the rocky moons that ideally support it is likely to do far more for total tonnage than anything else might.
Things like Refinery hubs (for the development level much more so than the weak links) and other constructions like Government Installations to normalise security can be built all over the
rest of the system so as not to waste a planet and then benefit not only the Orbis but also all the T1s as well.
(Obviously the system is going to hit "more than you can ever reasonably use yourself" with just the Orbis and the first surface outpost, but that's not the point)