...On a tad grimmer note, it seems awfully unlikely that any intelligent culture could survive prolonged exposure to another one, if the exposure is the result of interstellar expansion. The expanding culture would have two factors more or less dictating that it crush the encountered culture: barring incredible coincidences of timing, it has vastly
superior technology (as it sails from star to star and the other one at the time does not), and
it has found in itself the motivation to expand.
In the scenario where interstellar colonization indeed takes place, the inevitable end result would seem to be extermination of all (future) competition - until the original expanders start fighting themselves, as would equally inevitably happen in the STL scenario where ideas take tens of thousands of years to cross the galaxy.