It's similar to the L4D director, which is what gamemode zombie was based on.
You simply set down a spawn, in groups or scattered randomly, and then type /hidezombiespawns(must be host or admin for it to work) which will hide each spawn. You don't have to set what type of zombie on a spawn, but when the minigame starts, it should randomly play a tune, and at that point, the director will activate, spawning all kinds of zombies left and right.
If the director doesn't work, check the minigame menu, or event a brick that is triggered to start a director minigame event.
And you can have hidden guns, but what I'm talking about are guns in general.
It gets really annoying when one guy sits on top of an item waiting for it to spawn only to be sat on top of by the next guy...