RogueMage wrote:Way back in 2nd edition there was also fluff about why Tyranids and Genestealer Cults would fight each other
THIS one's easy... not everyone in a Genestealer cult may be under full control when the Tyranid fleet turns up to consume them. So yes, cults have been known to fight back against their would-be devourers, just not really to any great effect...
The only plausible explanation I can think of for Tyranids fighting each other is a lack of Hive Mind control. Say, if for some reason the Hive Mind's signal were "cut off" (although we know warp storms don't cut the mustard here) or if all Synapse creatures had been killed. In either case it's only on a small scale, it's not like entire Hive Fleets would be clawing at each other.
The Tau *sort of* have splinter factions, but they're kinda just beginning to show up - Shadowsun is a good example, and judging by the advance order description it looks like not everyone gets along with new character Darkstrider.
Eldar had splinter factions even before the Fall - remember how the Rangers and Exodites fled before Slaanesh's creation? You could also class Dark Eldar as another splinter, who themselves have many different houses vying for power in Comorragh.
Orks have clans, as mentioned, Chaos is *always* fighting itself whether it's warbands, Legions fighting (or even Legion infighting) or daemonic struggles. Necrons have different dynasties and the new Codex has examples of how they have fought each other, sometimes for power, sometimes because the re-awakening drove one side mad/faulty.
In short, there isn't really a major player in the
40k universe that DOESN'T have different factions within itself. Closest I can think of are the Grey Knights being a one-off special Chapter with no successors or similar Chapters, and they're still part of the Imperium.