This can be and has been done regularly, combining depleted units. It was SOP at many points in the Napoleonic wars by all sides, in the US Civil War, in WWI (happened several times to a single French regiment, the 157th. Ref the superb unit history “The Verdun Regiment”).
And in WW2, this too was SOP in the Wehrmacht, where remnants and odds and ends were routinely made into “kampfgruppe” on all fronts.
It certainly can work, but it depends on the army doctrine, state of training, leadership, etc.
:it depends on the army doctrine, state of training, leadership, etc.”
Can we agree that Russian army doctrine is not the greatest and their conscripts are not well trained and their leadership is iffy? Not to mention morale. There is word of mass disobedience and even some desertion.