You can not have both, meaning global settings along with individual settings for different users. Individual users can be found in Terminal User Manager and can be set up under the Config button. This will only work if you have the inherit user config turned on the Global Settings for Connection, Disconnect and Idle Timer in the Citrix Connection Configuration Tool. You could specify No Timeout globally and then have individual settings take precedence, but this would have to be administered to 500 users.
Scripting would work, I will post again if any of my current scripts would work for this.
Good Luck