The default Connection option for external Audio Mixer inputs, is Follow Video Source. When an audio input is set this way, it is assigned to a Local > Skype TX Caller channel. The audio control group is automatically assigned to govern incoming audio from that remote Skype caller.
It is not necessary to use an Aux audio bus to configure a special audio mix minus to return to the remote Skype caller in this case. The return audio for Skype TX is handled automatically, ensuring that (except when the TalkBack feature is engaged) the remote caller will receive a special mix comprised of the Master mix with the remote caller’s sound removed.