How Many Published Settings Fit in Planescape?

Spelljammer, Greyhawk, Ravenloft, and Forgotten Realms/Maztica/Kara-Tur/Al-Qadim. Inner Circle's Violet Dawn setting easily fits as well.

Dark Sun doesn't really fit with Planescape and the main canon, though. The history of the races is quite different, and their version of the Elemental Planes doesn't fit with the main canon, considering that most of them are dying.

Yeah, but it's kind of an in-joke amongst the planar community that people from Krynn are even more Clueless than the average prime. It was noted in one of the PS sourcebooks that the Dragonlance setting is a part of the big picture, it's just that the residents are kinda backwater types that think the Gray Waste of Hades is the Abyss and so on. It's not their fault, they're just a bit confused. This probably goes all the way back to the whole Weis/Hickman feud with TSR about using their baby within the greater scope of the D&D multiverse, something they seem vehemently against.

As far as the Forgotten Realms change that appears to break it away from the PS wheel - I simply ignore it. It adds nothing to the setting, other than to make things needlessly complicated and incompatible.

My players were never very knowledgable or enamored with the Planescape setting, but they're all well versed in and love several of the more traditional, terrestrial settings. So my campaign used PS stuff, but more as a backdrop to tie it all together and allow travel between the worlds. For the record I've used it to connect what I consider "The Big Five", or most iconic and important TSR/WoTC settings that really "make" D&D for me - Greyhawk, Dragonlance, Forgotten Realms, Ravenloft, Darksun.

I never learned enough about Eberron to try to fit it in with the others. If it insists on having a totally different cosmology that is so integral to the setting that it can't be compatible with the ones I mentioned above, than I'm happy to ignore it. If the cosmology is not that important to the feel and vibe of Eberron, than just use the Great Wheel and throw it on in there with the rest. See that's the beauty of an infinite prime material plane - you can put whatever you want in there, and if you're not happy with what you see, you can build your own planet from the ground up.

Many of the planes in Eberron can correspond to roughly to planes in the Great Wheel and beyond. Except for Xoriat which is the Far Realm and Dal Quor which is the Plane of Dreams, planes that aren't officially part of the Great Wheel cosmology.

It's cosmology is different because it's the Planar Orrery cosmology that was in the back of the 3e Manual of the Planes. But that can be worked into the Great Wheel as a simple waxing and waning of the connections of other planes with Eberron.