Google Talk and MXit work quite well together.
You can add a GTalk (@gmail.com) users to your MXit contacts to chat to them from your phone.
And similarly, you can add email@example.com contacts to your GTalk contact list.
Occasionally, there are issues between Google and MXit.
Like, some gmail accounts don’t want to receive messages from MXit, but other accounts do. Like, I have two gmail accounts, the one works perfectly, the other doesn’t. Google are obviously fixing up their issues still.
I think that different GMail user accounts are being load balanced over different areas/ips. This means that some accounts have problems accessing the MXit servers.