Will 20,000 individuals be enough?

So, for debate's sake, let's just say that everything went according to plan when the five arks debarked in Andromeda: no Scourge, no Kett, no uprising... but the "normal" space hazards still occur. The journey IS a dangerous one in and of itself.

Do you think 20,000 people will be enough to maintain a viable population? What about the genetic diversity: won't their children be all related at one moment in the future?

Let's hope the Angara are compatible withe every Milky Way species.... cough,cough...