How to add households from the gallery without making a duplicate save of them?

If you place the same Sims frequently, have you considered setting up your own starter save file?

Create the worlds as you want them - place whatever lots & additional townies you usually place - then save it as "Starter" or whatever you want to call it. From then on, when you want to start a new game, open Starter and immediately do a "Save as" to create a new save with all your townies already in it.