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do any of you know if something like this exists and if not how could I go about doing something like this.

thanks for any help you might be able to give me.

[Toughsox]

Some plugins have a cvar that allows twhat you are asking about. Like, AFK kicker only becomes active once 'X' players are on the server.
Can you be more specific in which plugins you are talking about?

[exvel]

This plugin will do the job: http://forums.alliedmods.net/showthread.php?p=590466
For disabling/enabling plugin use commands:
"sm plugins load <plugin_name>"
"sm plugins unload <plugin_name>"
OR
better way if plugin has its own cvar for disabling/enabling - use it!