It's important to have these in a separate bag for two reasons.
The first is that they require less cook time than the other ingredients and you don't want to ruin this beautiful dish by having overcooked seafood in it.
The second is because the acidity in the pineapple juice can begin to cook the seafood if it marinates together and that would be a culinary tragedy!
Personally, I like to serve mine with red pepper flakes and Parmesan cheese. This dish is chock full of flavour.