Remember seeing a L4D mod that included Xenomorphs. Looked like fun.
As for Haloizing L4D, it's an interesting concept. My biggest worry is that the Covenant races generally don't line up with the roles of the infected. It's a pet peeve of mine, to the point where I usually avoid simple reskin mods because they feel halfhearted or incomplete.

I got to admit, the following...

: Zombies..............................................................................................All
: Flood
:
: Tank....................................................................................................Hunter
:
: Charger.................................................................................................Brute

...are all pretty good matches.

: Hunter....................................................................................................Elite

But wouldn't a Skirmisher be a better fit?

: ?........................................................................................................Prophets
:
: ?................................................................................................Promethean
: Knight
:
: ?..........................................................................................................Didact

No need for those guys. =)