Gayashan,
The service id can be set as a regex (I think this is default). Thus, you can match anything; but you should make your service ids as specific as possible.
If you have 'multiple applications' at the same URL, CAS server will treat them as a single application. This could make single logout tricky because CAS will send a logout to only one URL.
Typically, the cas client is included as part of your application, rather than a gateway to your application(s).
Ray