For some reason it doesn't give me the option to insert my external mixer when I right click next to the bus like it does with the midi clips. Is it possible to do this? Thanks

Not quite sure what you mean by "insert my external mixer" but there is usually a control on the MIDI surface assigned to switching to busses. It might be marked as "Aux" rather than busses though.
 
If you are using ACT there is a "Controller Group" toggle switch in the properties page.

The short answer is yes.

By inserting my mixer I just meant activating my mixer to be used with the track. Clumsily worded. I'm assuming the term mixer or control surface can be used interchangeably. Maybe not though lol.
 
Nothing that says 'Aux' on my surface, but I'm sure there must be a way of switching. I'll have a play around.
 
 

I control buses with an Akai MPK 88 using ACT. Once the control surface is controlling tracks I have buttons set up to move between groups and track banks. Changing groups lets me jump top the buses and contorl them instead of the tracks. Hope that helps.