Anyway to set new models to open in mm by default with custom dimension style?
Units and dimension styles are document properties. Change those to your liking once in an empty file and save (or use the SaveAsTemplate command).
You can set a file to be your template file for new documents: see help here:
http://docs.mcneel.com/rhino/mac/help/en-us/index.htm#commands/new.htm
Thanks heaps,
Stu