There was a single maturity spawn (iirc Oratan Slasher Bandit was the mob in question) for the event held last year to win the starter packs. There have also been single maturity spawns on Calypso in the past for a number of events (e.g. Eomon, Argonaut Scouts) and there's a big spawn of Foul that's all Young. So there does appear to be some mechanism to do it. Maybe the Arkadia team thought that having pure spawns of only one maturity made it too easy, or maybe they preferred to have some crossover to encourage teamwork. I'd be inclined to agree with that thinking, though I haven't been on Arkadia to see any of these new spawns yet.