Sorry about the delayed response. But your suggestion sounds reasonable. I'm planning on adding some more features to the app again, and adding a third unit type is not a big problem, but not entirely trivial either. It will require changing the preferences layout a bit, but I have been thinking of tweaking it somewhat anyway.
The feature will first appear in a beta version, hopefully within a few weeks. So if you are not a beta tester for the app yet, you may want to join beta tester group through the app listing in Play Store. Currently both beta and release versions of Custom Maps are exactly the same.