Hi there @mantravine
Just took a look to your video showing your issue, to be able to record incoming Midi from another instrument onto a new Midi or Audio track in Live, you first need to select your input channel like you did, in this case choose your track with "Serum" ( 1 ) , then make sure to set the monitoring to "IN"( 2 ).
If you already have existing Midi events or midi clips on the source track ( here Serum track ) you just need to arm the destination track for record, if you want to record the Midi as you play via a keyboard/controller, just arm both track for record and be sure to select the source ( serum ) track to be the active one ( 3 ).
Whether you're recording the Midi Out to a new Midi or a a new Audio track, the principle is the same.
Only for certain Midi instruments or arpeggio or sequencer generators like X-fer Cthulhu or Audiomodern Riffer (i.e) you will need to replace the default "Post Fx" setting by the track containing the source instrument again, but to record Midi out from Serum, just follow the simple first I/O routing.
Check the forums in order to visualize the screen shot giving you a example.
Hope that helps, cheers !