The options for live music at civil ceremonies depend on three things, the room available, the budget available and the mood that you would like to create.
Some rooms that are licensed for civil weddings are so small it would be difficult to squeeze in one harpist let alone a string quartet.
Of course, if cost is not an issue, a string quartet may provide a very classical and sophisticated atmosphere whereas a flute and classical guitar will have a very wide range of styles and probably be less expensive.
The mood that the instruments create is also an issue and this will probably be what determines the final choice of the ensemble.
