The funny thing about this is that I did everything completely wrong and backwards, but in the end it tasted pretty good. I actually started by steaming some mussels in white wine and butter with onion and garlic. When they were done, I removed them from the liquid and then used the cooking liquid as the base for the soup. I chopped up some linguica sausage and added it, along with some chicken stock. I chopped up some kale pretty finely and added that, then let it cook for 5-7 minutes. In went some shrimp and some chopped-up cod, a splash of sherry and a little salt and pepper. I let it cook for about 5 more minutes, until the fish was cooked through.
Wait, you're saying...what happened to the mussels? Funny thing about that, actually. I kept picking at them as I was cooking everything else, because they were sitting right there in front of me and I was hungry. By the time I got to the end of the soup-making, they were gone. It's ok, though...they gave the stock plenty of their tasty goodness, and they made an excellent snack!