We announced a new communications product, Hangouts, in May 2013. Hangouts will replace Google Talk and does not support XMPP.
Yes. In fact, it's required. Google Talk now hosts users from several different domains--don't assume that it will always be gmail.com. Your client should allow a user to enter their full username@domain information when they sign in. If a user does not enter their domain, you should display an error message. (As an alternative, or to make your application more robust against users entering an incorrect domain, you could implement the JID Domain Discovery XMPP extension.)