You can select elements you've instantiated by dragging and dropping them from the library onto the stage in two ways:
Directly clicking on the shape, on the stage.
Clicking on the shape's name on the timeline's properties panel. This is very useful when you have multiple slices on the stage, especially if they are stacked.
3. Locking the group or layer that you don't want to select, click and drag on the stage to select all, and you'll see that the locked layer/group doesn't get selected.
After doing all the adjustments you need to your animation (e.g. grouping), you can unlock the layer.
TIP: You can lock layers by clicking on the [lock] icon below the asset's name in the timeline. This also disables all "pointer events" on that element.