That's one of those basic plot points that I feel should be more obvious, but SC1 really makes it unclear how long they've been in contact. Like Fenix's statement after the first battle at Antioch or whatever, when he says something like "You know executor, although we two have marched across hundreds of worlds together, I never imagined that we would be fighting on Aiur. The zerg are indeed worthy foes." Kinda sounds like they haven't actually been fighting the zerg very long. And then on Shakuras, Raszagal and the other dark templar have no idea what the zerg are. They've literally never seen them before.
