A District Commissioner is likely not the final authority on unit number assignment.
If you're on the verge of starting a new unit, your council should have assigned you a District Executive ("DE") to help you with the process. This professional staff member should be tripping over themselves to answer your questions and resolve any roadblocks you face. Picking a unit number should, to them, be almost a non-issue. As long as the number isn't already in use by someone else in your area, I'd be shocked if they tried to enforce some arbitrary numbering scheme. Yes, there may be some computer system backend code that uses some other ID number for your unit, but that you should really have to deal with on day to day basis.