What i need though is a reason why Farsight would be in desperate need to colonize a planet
A small and endagered alien species is bio-harvested from the humans for their useful genetic material. being a somewhat advanced species and an useful one for their knowledge, they ask for tau's help.
they become part of the Tau Empire, trying to be saved from the Emperor's lackeys. The tau, understanding this race value and knowledge (and also discovering ruins of this civilization with many archeotech goods) deide that the crittersa re worth fighting for and the planet needs to be explored in full. So they attack. The empire strikes back. The tau are going to be pushed back but the eldar arrives. this happens because the alien species are in truth eldar creations, gifted with many useful techno-trinkets and knowledge and genetical mirabilia. Eldars need this species to survive in order to complete a genetic experiment of their own (or some other crazy eldar plot). A tau-eldar alliance is then forged, in the defence of this small alien race and for the taking of the planet.
EDIT: sorry for my bad english.