Another technique is to mimic that used by Napoleonic battalions (and probably as far back as Greek phalanxes if not further). 

Frontage is generally maintained constant, and as a man falls (in any rank except the back) he is replaced by the man directly behind him stepping forward. The back rank is shuffled by the NCO's to ensure a few men at the extreme of each flank, and otherwise to fill in evenly. 

The frontage is only reduced when the back rank falls below pre-specified densities. Likewise, when the back rank is overfull the extras first start filling in an additional rank from both flanks in, and then the frontage is increased.

When changing frontage, I suggest having your bots file out from the back rank to both flanks when increasing frontage, and filing in from both flanks to the back rank when reducing frontage.

If I am correct in inferring that you are looking for a "military" impression, and having your bot organizations look like phalanxes, I believe this ordered re-arrangement is a better way to achieve that end.