The short answer is that, like everything else in the Guard, it varies wildly. Some have massive numbers of troops (one of the Tallarn regiments during the Taros Campaign had more than 10,000 troops), others have relatively small numbers (~1000 for another regiment in that same campaign).
Similarly, the command structure itself is all over the place, as your examples illustrate nicely.
"You've all been selected for this mission because you each have a special skill. Professor Hawking, John Leslie, Phil Neville, the Wu-Tang Clan, Usher, the Sugar Puffs Monster and Daniel Day-Lewis! Welcome to Operation MindFuck!"