Well, what do you mean by unit shoving? If you're thinking of redeployment, then mass-scale redeployments should be quite difficult to carry out, unless they have been carefully prepared (as in pre-designated prior to battle start). Smaller scale redeployments will not be too difficult, except that redeploying troops in the midst of combat is extremely difficult.
It is always possible to extract a force from the battlefield itself (in fact, it will usually be the smartest thing for the General to do, if the battle looks to be turning against you). But this phase only models the actual retreat from the battlefield and to the army's camp.
After the battle, the losing army will usually try to withdraw from the area, and whether this succeeds depends very much on the ratio and the state of the enemy's cavalry compared to one's own, what state the troop's one extracts are in, and the commander's abilities. Good commanders will tend to be able to retreat after a battle with their armies more or less intact; bad commander's will likely see their armies destroyed by desertion or their enemy's determined pursuit.
Though I've never played it (only heard of it), I think the game system more closely resembles Steve Jackson's "Rome at War" system rather than SPQR. But of course, inspiration is taken from all kinds of sources.
