This would be enough for the fields. No need for it to go all the way around the crops. Remember that water will irrigate up to 4 blocks away. I can't think of a city that places their farms or crops inside their city, or their forests for timbering.
How about, instead of just one preset house construction, you mod in around three or four? Two at least. And if you want to save on space you could manually construct an inn that would house multiple colonists, or even two or three story houses. Or, larger single story houses. OR, again, you construct the houses for the colonists (being 5x5x3 with a roof at least) however big you want.
From the other thread. And on that preset construction thing, I meant that the npc builds 1 of 2 or more preset houses/workshops randomly.
Nath does seem right about that reproduction thing though, too much too soon. I really want it in though. Seems much more natural.
And as a last note: A city CANNOT be with just 20 people. That's a tiny town at most.