Z and Barbatus are standing side by side in formation. The platoon then executes a right face and Barbatus encourages Z to move up putting him side by side with Barbatus again. While this is convenient for conversation, it wouldn't have worked because whomever had been in front of Barbatus in formation would now be in that spot.
Water changes its scale at various times during the movie. Sometimes, it features surface tension consistent with the scale of the ants, then at others acts like water on the scale of humans (ie. the flood scene). The directors acknowledge this discrepancy.