I think have some initial infected maybe wouldn't work as there would be problems of making balanced while still fun and stoping powergaming/metagaming. Plus it wouldn't be too hard for the start zombies to fail quickly.
Maybe it would be better to have some sort of start event that is a large number of zombies on a shuttle that arrive at the station, maybe there could be a few player controlled ones and if the player zombies die they just get a new one. Suddenly it gets very interesting as while the zombies are slow they would already have their greatest advantage-numbers!