Skaven Clans are not that different from one another aesthetically or religiously, they all worship the same god. As for the moonclan, spiderfang and troggoths they were not established in the Realms at all. They could have did anything they want with them. And they all worship the bad moon.
The three dwarf factions however are pretty much established already in the realms. Besides being Dwarfs, they have nothing in common aesthetically, religiously.
One the factions builds their holds under the free cities and are responsible for building and architecture, they even hold seats upon the grand conclave.
Another are a culture of mercenaries that would even fight for chaos (at the moment anyway) hammer runes made of molten gold into their flesh.
Another live in great cities in the sky and fly around in the flying ships, they don't even believe in the gods anymore.
I don't see these factions merging and there's no reason why they should.