I think that suggestion is purely UX upside with no additional security risk. Validating the input password on the login page should expose exactly the same am

I think that suggestion is purely UX upside with no additional security risk. Validating the input password on the login page should expose exactly the same amount of information as validating it on your signup page. A remotely-motivated attacker will harvest all the constraints he can from your signup page. Trying to hide them on the login page buys you nothing.