No, video conferencing is not yet supported.
No, currently Coccinella only supports IAX, which is not (yet) supported by Google Talk. So, you cannot yet receive calls from or make a call to people using the Google Talk client.
Yes, since version 0.96.19 (current development version) you can connect your Coccinella client to an Asterisk PBX, using the IAX2 protocol. On your Asterisk deployment, configure an IAX2 user similar to the following example:
 type=friend username=100 secret=MyPassword context=default host=dynamic qualify=yes callerid= John Doe
Subsequently, enter the following information in Coccinella in the IAX Account preferences: