First, a better auto-pluralize algorithm for English would add "es" when the word ends with an "s" (but it would still be wrong in this case).

I suggest a field in the editor that's populated with the automatic plural of the singular form as I type it, but can be edited by the user if they don't like it.

You might also want to hardcode a few common unit names that people are likely to use and that are hard to automatically pluralize, like "swordsman", "bowman", "man at arms", etc, and have hand-written plurals for them.