The Ships Stats page lists the missile launchers as being "Marsden 683" racks. Does this mean you can also replace these with "Marsden 616" or "Marsden 625" racks in order to equip a small number of torpedoes instead?

We aim to make them interchangeable. The blockout for them currently meets the metrics for allowing interchangeability but things may change as the ship moves through the pipeline. For more information on Torpedoes vs Missiles please check out this Shipyard post which goes into detail on how the two types can be mixed together.