You can copy items from multiple layers at a time in Photoshop by simply adding the Shift key to Command + C or selecting Copy from the Edit menu. Now when you paste, it will give you results as if you had flattened all your layers before making the copy command.
Layer Sets in Photoshop are a great way to organize your layers but many times you will need to merge a set to work on its component layers as a merged, single layer. To do this, activate a layer set, go to the pop-out arrow in the upper right corner of the Layers palette, and drag down to Merge Layer Set. This will replace the set with a merged composite of the set’s component layers. But sometimes I want to get a merged version of a set to work with and maintain the set with its component layers intact. To do this, follow the same sequence above but hold down the Option key before you release over Merge Layer Set. This will merge the set out into a new layer above the existing layer set rather than replacing it.