Salt is the curing/preserving agent in my recipe.
Tender Quick is salt with other agents to aid in preservation, color and flavor enhancement, and would be fine to use for this recipe. Keep in mind,
Tender Quick is NOT a meat tenderizer. Do not deviate from the amount of salt called for in any dry cured sausage recipe. Too little and the meat will not cure properly, and too much will make a salty sausage that nobody will want to eat.
I use only what is listed above in the recipe. Some folks like a spicy sausage and add crushed red pepper or garlic or fennel seed (or all the above) to the mix, but then you're starting to cross the recipe line from a Sopressata from the Tuscany region to a hot Calabrese style sausage from the South if Italy. Each region has its favored recipe, and you can find recipes for all of them online. You can pretty much add whatever spices or herbs that you like to sausage to satisfy your particular palette, as long as you adhere to the principles of dry curing.
The Prague powder prevents bad bacteria from attacking the sausage while it slowly dries from the outside in.