Probably no need to chime in, but whoever came up with that "rule" was not from New England. Fish chowder is typically made from cod, pollock, and other white fish, with onions and potatoes in a base of cream and milk. If you've got access to them, it is also common to add lobster meat, clam bodies, etc.
Chowder is a favorite "warm up" meal in cold weather, and a great draw for tourists in all weather. I don't know if they are still in operation, but McLeod's in Bucksport, and the Snow Squall in South Portland were always high on my list for traditional fish chowders when I was doing tech service work for paper mills. Unless one is lactose-intolerant or allergic to shellfish, the only "risk" of eating good fish chowder is a chance of over-eating.