If you want fast, simple, easy to teach zombie rules, you can try the FREE and EXCELLENT AKULA'S rules:
http://akulasrules.blogspot.mx/2011/01/akulas-rules-skirmish-edition-arse.htmlThese are generic rules, and has a mechanism to get more and more zombies each round if you so desire with "Zombie spawn points".
It even considers several "special" zombies with special activation and effects (fast, tank, howler, horde).
But please note that AKULA don't explain everything in his rules ON PURPOSE, in order to keep them short and simple (just 2 pages), so a little experimentation may be needed.
Fortunately Akula is a member of this very forum so you can ask him directly if needed (I did).
I have tryed his rules and really delivers the "zombie survival" feel with simple and elegant mechanics, even an inexperienced player can grasp after just a few minutes.
Hope that helps.