@Iakus: "Spreading" to another galaxy is not what seems to be going on here.
I really don't believe the Milky Way is going to be flourishing at the beginning of this game and these colonials/explorers are just an extension.
However they got there - I'll wager the trip was one way, their "civilization" will be confined to a flotilla like caravan of ships (or that massive picture that's floating around).
I don't get the feeling this is just some casual day trip to the Andromeda galaxy.
And why wouldn't the Reapers be able to get there? The question is, why would they come? The Reapers answer the Citadel dinner bell do they not? They came to the Milky Way because the thing was being used (for hundreds of years if I'm not mistaken).
So, these survivors get to the Andromeda galaxy... there's no need for the Reapers to follow... at all. IF they still exist (and I believe they will) they're going to be the "boogey men" between galaxies... waiting for someone to trip the dinner bell (I believe there will be an Andromeda Citadel)
The Reapers don't magically detect advanced civilization... the whole point of the Citadel is that the mindset of such civilizations that would reach space is careless and would gobble up the trojan horse Citadel tech... thereby tripping the dinner bell.
Survivors would know what it is... they could quarantine such a place - the Reapers would never return to the Andromeda galaxy.