Quick take
When iron is significant, remove or reduce iron before the softener. Then size the softener from hardness, household flow and remaining iron load.
Why a softener alone can struggle
Ion-exchange softeners can handle small amounts of dissolved iron, but higher iron loads foul resin and increase regeneration demand.
Preferred treatment order
A common train is sediment pretreatment if needed, dedicated iron/sulfur/manganese treatment, then a softener for hardness. UV or RO may follow for separate microbial or drinking-water concerns.
Combination systems
Combined iron-filter and softener packages can simplify purchasing, but the individual stages still need to match the measured water chemistry and peak flow.