It was my wrong choice of word, as there is no wrong in creating your own food. You can put ketchup in pho if you wanted to and it's still a no wrong if you like it
Crushed peanuts is on the table but should be meant for other "dry" dishes/appetizers. Peanuts generally don't go into broth, but you could do it if you like to. How about some soy sauce along with it?
Shrimps (or seafood in general) don't usually go with beef in Vietnamese dish, although they do go with chicken/pork/vege base broth soups all the time. (I'm ready to stand corrected here with some examples of classic Vietnamese dish with both seafood and beef in it.) Remember where I said every restaurant has its own twist to Pho? Making seafood available to Pho is a good example.
A general misconception (in non-Vietnamese) is also known that all noodle soups at a Vietnamese restaurant is Pho, which is not correct. There are a lot of them soups in Vietnamese cookbook because they are not usually served as an appetizer but mostly a main course.