It is stated in the Esposito/Elting Atlas. Why do you think they were on the wrong side of the river and marshes when the bridge was prematurely destroyed. And those three commands were not too much or too many to form the rearguard as they were expected to delay the allied army. You are familiar with what a rear guard is supposed to do? And it is usually more, as at Leipzig, than a detachment left in contact.