This might just be the most profound thing I've seen posted in a Wandering Monsters column.I think there's a tremendous value in allowing DMs and world designers the freedom to design a cosmological system that suits the exact needs of a particular campaign. But this approach has its pitfalls as well.
Probably the biggest danger is in eroding the things that everyone knows about D&D—the D&D intellectual property, to put it in legal terms.
This seems to me a better argument against an over-setting: obviously, a huge variety can fit within a single setting, as in the real world. But doesn't it allow for even more variety to also allow for other, more thematically focused (even if that makes them less "realistic") settings? Shoehorning all of them into the over-setting necessarily either limits the possible premises, or reveals them as somehow mistaken, limited, &c.D&D has always been a blender game of widely varied contributions, because it came from a widely varied world.
Not to pick a nit, but they are actually cut down to infinite size. Infinity of physical extent is, to be fair, something very, very many people don't really seem to grok. If the Astral sea is infinite, there is no real reason that some of the realms within it should not be infinite (in a limited number of dimensions) also. If you have an infinity of space to spread out into, you can spread out into it infinitely.Furthermore, it'd be nice to have the outer planes take a leaf from 4th Edition's book and be cut down to a finite size.
...the reality is that it's not actually very hard to reconcile even vastly different cosmologies. As I've mentioned before, the Great Wheel cosmology doesn't model an objectively verifiable truth. There isn't a being in the multiverse, except maybe an Overgod figure like Ao (and he's not talking), who can look down and see the planes in their arrangement as we look at a diagram in a book. Is the plane of Celestia sandwiched between Bytopia and Arcadia? Who can say? The only way to get from one to another is through a portal anyway, so for all anyone knows, that portal could be crossing a thin planar boundary, hopping to a different branch in a cosmic tree, or traversing incredible distances across an Astral Sea.
For that matter, is there actually a place called Celestia? A lot of lawful good deities seem to have realms with quite a bit in common—steep mountain slopes, archons all over the place, an air of beneficence to the place—but are they physically connected? Maybe. Maybe not.
For the purposes of your campaign, it doesn't matter. It doesn't matter if you talk about Celestia or about the Seven Heavens or about the distinct divine realms of Green Fields, Dwarfhome, and the House of the Triad.
Why do you honestly need a guide? Creating a cosmology would be considered an advanced level task amd by then, you shouldn't need a book to tell you how.