Moneybags figured out how to get single soldier units, by using elephants with "soldiers 1" in the spawn. Before then it was not possible at all so until then TATW's Balrog was actually 8 Balrogs (or whatever the number was). The "one man unit fix" was to use Moneybags' findings to allow a single Balrog and Sauron. It was then found that sometimes this method gives us 3 Balrogs/Saurons ... to that there is no fix yet, as far as I know.
Moneybags' tutorial: http://www.twcenter.net/forums/showt...ts-in-Campaign
Cause of the "3 Sauron" issue (later in that thread): http://www.twcenter.net/forums/showt...1#post11982551
That's right.
EDIT: to clarify: the Balrogs are spawned under two different circumstances and in two different ways. One way does not allow the "soldiers 1" parameter so cannot create a single man unit. It has nothing to do with it being under the player's control.