"Tyranids fight with each other now. Hive Fleets compete with each other and clash over food on planets. The Hive Mind is still stated to be directing the overall Tyranid race regardless of this, this is probably the stupidest thing in the Codex."
This had been addressed in the previous Tyranid Codex. Their infighting is in no way "stupidest thing in the codex". They fight and compete with one another to find the strongest genes and mutations - the "defeated" fleet is consumed by the winner and ALL the biomass is used to improve the Tyranid species as a whole. Tyranids do not waste their own - they eat and re-purpose them in an improved form.
'Nids suffer no losses from such "war". It's more akin to bio research, as nothing is lost, but a lot is gained, in exchange for time. Survival of the fittest for the benefit of all Tyranids. While other races kill each other, 'Nids just keep growing better and better. Makes me wish that GW did not alter the fluff for the C'Tan - having "The Outsider" as a potential Great Hive Mind was sooo neat.
In regards to Tau, I've read somewhere previously that GW introduced the idea that Emperor was searching for aliens that are immune to Chaos influence, and imparted onto Astartes to continue the search. Tau are these aliens. Now the Marines are protecting them, as they are the mankind's hope to push back the Ruinous Powers back into the Warp and seal them there.
Hope is good.
I'm actually in the middle of building an army that is the herald of Emperor's return, composed of Custodes and the Terran guard(Allie rules FTW). Oh and an Avatar of "The Man" himself. Great Crusade II: the Sequel
