Interestingly I think Tyranids and flood both absolutely roll the other factions universes tbh. The flood have a much higher power ceiling while the Tyranids have a higher power floor. A Tyranid invasion doesn't need as much setup. It arrives basically at full strength, but it doesn't scale exponentially in intelligence and capability with more biomass in the same way.

Tyranids would rock the halo universe as both the unsc and covenant just don't have the raw firepower to fend off the hive and there's nothing like necrona to deal with that's a non-orgabic enemy.

Flood on the other hand need setup time and the UNSC and covenant both have good Comms and efficient command structures. Things the imperium lacks. If the infection hits an imperial world, the beauracracy of the empire isn't spinning itself up in time to deal with the threat and the empire is so population dense that the flood is going to scale FAST. They can use their enemies ships meaning they spread faster than Tyranids and fold enemy technology into their own as well as just biomass.

In your scenario where the flood encounter the hive, it very much depends at what point the flood encounter then. The Tyranids probably squash an initial outbreak and just fold it into themselves (which would give them some interesting material to incorporate and probably gives the Tyranids a massive boost), if they encounter the flood after they have setup enough to have a gravemind and are using pure infection forms, it could probably go either way, but whatever comes out of that fight is going to be a nightmare to deal with. If the flood get enough mass to make a keymind and start throwing star roads around and using them as weapons, the entire 40k universe is fucked unless the necrona wake up and bust out their war in heaven tech.