1) codec can support 6 channel decoding, but for encoding, I'm not sure that, I can reply to you later when I confirmed it.
2) for this plugins, I think you can use the command "gst-inpect-1.0 mfw_mp3encode ", then you can get detailed information for it.
because I'm out of office, couldn't send the detailed information to you ,I think you can use it quickly.