1. You can use any kind of seafood you prefer in this dish. Oysters (drained) are good if you like oysters. (I add 1/2 cup of the oyster liquid with the oysters). A few other suggestions are shelled crawdads (not really seafood), clams, octopus, and while rather pricey, lobster and cram meat are really delicious in the dish. The ones I included are just my favorites. Use your imagination to make the recipe suit you.
2. You can increase or decrease the amount of spaghetti or Alfredo sauce to suit your taste. I personally like plenty of sauce with my pasta. I like to dunk my bread in the sauce. I also personally think all pasta dishes are better the second day after they have had ample time for the pasta to soak up some of the sauce flovors.
3. As with most of my recipes you can leave ingredients you don't like out or add some you do like. For instance if you like your pasta spicy add more red pepper flakes or some of your favorite hot sauce. Depending on how spicy you want it you could also add some finely chopped jalapeno or habanero peppers (with or without the seeds). If you don't care for mushrooms, or maybe red pepper flakes, then leave them out. If you like it tell me so I can try your version.