They can use cavalry, but then they should use the cavalry wisely, there's no need to use cavalry to distract horses, you should use cavalry to attack archers, or to circle around and charge the ennemy infantry in the rear when they meet your battle lines. Usually cavalry like to charge rhodoks nords head on, so keep cavalry in the back, behind the lines, and when the enemy cav has been killed by the pike/shield wall, then the cav should circle round the field and either take out archers, or ttack infantry in the rear when they engage your pike/shieldwall.
that's how Nord/Rhodok cav should be used, and for that yes they do need their cavalry