Ok I'm jumping into this.
I remember like crazy playing through Resident Evil series, and also I hated the flood like the plague in the 1600's.
A Ninja would exposed himself while killing the space pirate. The Flood would take notice of this and "Flood" the Ninja until they killed and consumed the ninja. FLOOD NINJA! if there were any unconsumed bodies lying around and by some unknown they came back as Zombies. It would be an ongoing conflict between Flood and Zombies because the flood would consume the Zombies and the zombie would do the same to the Flood, I.E. ONE BIG STALEMATE! No winners.
It also depends on what type of zombie are we talking about here; T-Virus, George Romero series: Dawn, Day, and Night of the living dead, or Mystical like D&D or some form of magic involved, or Dead space(which I think are more like the Flood) and RE Parasites (Flood-like zombies).
This has been another random quip from 1014, thank you and enjoy!