PMFJI but I want to point out...
  • Macintosh HD is your boot volume and is 120 GB in size out of which you are curretnluy using only a little over 13 GB
  • Allowing for some overhead a clone partition/volume of 150 GB would be more than adequate
  • Your Sierra partition/volume on the WD My Passport at 333 GB is twice what you should need for a backup clone
  • Artie is correct that changing partition sizes is possible, but there are rules on which partitions can be resized non-destructively, and even if you follow all the rules correctly there is still a finite risk of losing data.
Since you no longer want Mountain Lion and rather than over-complicating the process and your d=current partitions are more than adequate for clone images why not…
  1. Erase the Mountain Lion volume
  2. Rename it something like Archives or Installers
  3. Use the renamed partition to store dowloaded application and OS install .dmg file.
If you were operating much closer to capacity on the partitions then more thought and planning would definitely be in order, but in this case it does not appear to be worth the additional effort.

If we knew what it was we were doing, it wouldn't be called research, would it?

— Albert Einstein