Well for my game An Instinctive Fear I did a survival mode, and its quite fun and challenging. Because of how FPSC characters can easily get stuck I designed my arena to be simple in the first place - almost no entities in the first place and a roughly diamond shape. The level design meant that there was a point where it would become too much. The player has enough room to run around, but will be cut off eventually. If you design your area small but not to small to begin with it sets you up easier for the future.
If the level is smaller, simpler or tighter less enemies are needed to make it challenging.
Look at the original call of duty zombies map, it was small and simple enough, yet very fun and challenging. For FPSC though the level design might be too ambitious for the AI to navigate. If that's the case an arena styled level like mine in survival suits FPSC possibly the best, you will never run into stupid AI problems like being stuck on a bucket or a piece of furniture.
The map itself doesn't have to be smaller, but funnel things down and simplify them. Corridors are an example, they are narrow and can cut off your player. Creating tense gameplay without hundreds of enemies. And so if there is less room (like a corridor) you can't really rapetrain them effectively. Because, as a developer you would never have a dead end, a zombie spawn would be there.
If you're map is small enough then you can spawn enemies within the limits of FPSC, and their stats that you alter will have a bigger impact. In my Survival mode the strength of a character increased over 100 health to the latest time the player was expected to survive until.
I don't know if you have already, but consider reading Gencheff's Zombie system thread, its worth a read.
http://forum.thegamecreators.com/?m=forum_view&t=178003&b=23
And I don't know if you have read it but in my W.I.P thread for An Instinctive Fear I talk to the Nerevar about how I did Survival, and gave away some of my scripts.
Few - end of post
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