Ok here's how I won three Alien matches in a row now:
1) One player, and not more of the alien team evolves to Gorge, the rest stays Skulk and rushes the marine team, disturbing them in their build-up phase.
2) The Gorge builds 2 ressource nodes at the 2 possible hive locations, then one ressource node at an external position close to a hive. He also tries to convince his team to defend ressource towers that are under attack.
3) After he built 3 ressource towers the Gorge: Saves up ressources till 80 -> grows 2nd hive -> builds another ressource tower close to a hive -> saves up ressources till 80 again -> grows 3rd hive
4) While his team continues annoying the marines as lerks / skulks / soon fades (DON'T just sit in your base, "defending"), the Gorge builds 3 defensive, one movement and one sensory chamber at every hive. When he finishes that the whole team evolves to Onos gets carapace / cloaking / celerity and wanders around the map, destroying marine ressource nodes whereever possible. Now one or two of the onos evolve back to lerk with adrenaline / regenration and the whole team rushes the marine main base, semi-invicible thanks to the umbra.
Of course this assumes you can convince your team into not all changing to Gorge, defending the ressource towers at the hive positions (when they're down and the marines build turrets there = gg) and in general act in some productive way.