As history is witness, "united towards" doesn't work in large scale. It should be "united against". Human need an enemy to get united against. Alien invasion, asteroid collusion might be such thing. Otherwise no matter what, humanity will find one reason or another to form groups to fight one another.

Therefore humans are unlikely to get united to colonize a new planet few light years away. That's not in human nature.

Also check Monkey Sphere: http://en.wikipedia.org/wiki/Dunbars_number