If we enable the SAML SSO, will the users still be able to use their current user name and password to sign in as an alternative way? Or they will be redirected to SAML sign in page?
It depends on the user role. If you have allowed them to by-pass SSO, they can login. Otherwise you have to provide them the interface to your system to login with SSO. It will not automatically redirects you to a SAML sign in page.
Like we use Mircosoft Azure to manage SSO. So all users other than Admin, have to sign marketo by login to myapps.microsoft.com.