The reason given for not eating fish and meat together is due to Sakana. In current times it does not seem to be a Sakana, (Nishtanu HaTeva). Would you be allowed to eat them together? If not - why not / If yes - why yes?
See the discussion here. The Magen Avraham shrugs his shoulders as to why this prohibition became carved-in-stone halacha, but once it did, we accept it as such.