I know the rules are not written "THIS" way, but I would suggest the following alteration to make up for a poorly worded rule.
"When a Detachment compares its ranks to the enemy's for purposes of determining Steadfast, the Detachment may count the Parent unit's ranks instead of its own."
(IE: Detachment of 10 Swordsmen in 5x2 formation lose combat to 15 Orcs in 5x3 formation; The Swordsmen's Parent unit has 5 ranks, so the Detachment counts its own ranks as being 5 and therefore is Steadfast for this round of lost combat)
........................
Now a curious thing is Skirmishers in Woods. Skirmishers with a majority of models in a forest gain Steadfast, so if the Parent unit is an Archer unit and they are in woods, would all detachments gain Steadfast too? This would be confusing when a ranked up Halberd Detachment is in Woods and so is the Parent Archers unit since the rules suggest that the Detachment would both be Steadfast AND lose Steadfast!