[coremoff]

it's a bit more involved than that, IIRC (been a while) - the civilisations involved typically migrate around the galaxy, entierly abandoning entire arms of the milky way and moving to another; agreements with other non-compatible civilisations mean that those that remain exist in an extremely hostile environment (the wars between the oxygen and hydrogen breathers, in particular, were notable until they managed to come to this arrangement).

There are also significant numbers of extremely agressive and militaristic civilisations, mostly held back by the laws and customs of setting, who would gleefully seek out and destroy anyone not following along.