...My hardshell will never fit over everything, nor do I need it to. The down jacket I am looking at will be water resistant as well.
I have several "hardshells" for different conditions. My lightest is a single-ply Marmot Precip, and it's the one I use either in summer conditions or on lower peaks in the West because it's rarely cold enough to warrant anything heavier - no polar air masses to deal with. On the other end are a couple of 3-ply, expedition-grade jackets for subzero, windy conditions. Regardless of 1-, 2- or 3-ply, they're sized to fit over layers, as that's their function, at least for me.
As for a water-resistant down jacket - if it's warm enough to rain, you probably don't need a down jacket. Down gets its warmth from loft, and that means the fabric is probably wind-resistant but probably not water-resistant.
