One case I think you left out (although I may have misread what you wrote):
You can talk between GTalk and a federated XMPP server if the the GTalk user is logged in via XMPP. So if I send a message from my work XMPP server to my google address, it shows up in Adium, but not in the hangouts client.
It's almost as if Google's XMPP network is separate from hangouts, and they only allow messages directly to/from the Google XMPP to propagate between them.
You can talk between GTalk and a federated XMPP server if the the GTalk user is logged in via XMPP. So if I send a message from my work XMPP server to my google address, it shows up in Adium, but not in the hangouts client.
It's almost as if Google's XMPP network is separate from hangouts, and they only allow messages directly to/from the Google XMPP to propagate between them.