I don't think i've read about "the shadow". But as far as the descent (opposite of your ascendent) thing, I see it is the people that you are attracted or attract into your life and are there to complete deficiencies. For example, I have Scorpio Asc so my Decendent is Taurus. That doesn't mean I want or am attracted to or should go out with a Taurus but that I'm looking for the qualities of a Taurus.
So I'm (or was) a bit flaky, irresponsible, took every thing light hearted, flitted around but my ex was dependable, responsible, stable. I loved to go out and he loved to go in. I love to be social and he's happy to be alone. So with me he went out more and I stayed in more. He made more friends and I made better more deep friendships.
We balanced each other out. He made me more stable and I made him more adventurous. He was not a Taurus, but a Leo
Maybe your shadow isn't to do with a partner. I don't know if it's always to do with partnership/marriage. I have Saturn there. So I think for me it is. But for you maybe it's some other part of your personality/life. I don't know enough to say.
Try to see the qualities in capricorn that you don't have that you might want to work at having.
Again, it's not about people in that sign. But the qualities of that sign that you might be lacking.