When the foot dismount, the mounts are on a cavalry or camelry sized base and there is no reason to think that this would change when the riders get on the horses/camels. Therefore any other size of base is likely to be viewed as suspicious and possibly challenged.
However, a Hd with a Cv/Cm base behind, would occupy the same footprint as a double cavalry base. Therefore you could satisfy most rule lawyers by saying that these bases represent a Hd on foot plus the empty mounts behind them, and you are just depicting them as all mounted for aesthetic reasons.
