For the difference between live and shoutcast please see here,
The link states:
(Optional): If your application will only stream to Flash RTMP clients, in [install-dir]/conf/live/Application.xml, you can set the Streams/StreamType property to:
If you do this, then you won't need to use StreamManager (see Publishing the stream). The first Flash client that plays the stream will trigger Wowza Media Server to start re-streaming.
Also, in [install-dir]/conf/live/Application.xml, make sure that the Streams/LiveStreamPacketizers property is empty:
You're correct, if they find out the RTMP path they could use it. Without a custom module you can't block any other shoutcast streams being published through your application.
If I go for "live" application and use alias.stream ( mediacaster ); That wont prevent that if somebody knows RTMP path, could use it, right?