If the Video drop-down list is grayed out as soon as you run iCamSource, that probably means you have Auto-Start enabled. Please disable that option for now.
If the Video drop-down list is grayed out, that means that iCamSource has been started and is capturing video from your webcam. In order to changed the setting, you'll need to stop iCamSource by clicking the Stop button, or by quitting and relaunching iCamSource when the Auto-Start option is disabled. Then you should be able to select whatever webcam you want to use.
If you want to connect multiple cameras to the same computer, you'll need to run multiple instances of iCamSource on the computer (one for each camera).
To do that on Windows, simply run more than one instance of the iCamSource, enter the same login and password, and set up a different camera on each instance of iCamSource.
In order to watch all of the cameras on your iPhone at the same time, make sure that all instances of iCamSource have the exact same login and password entered.