Hi Danny,
a bit more background info would be helpful (such as ABAP or JAVA system, what is the IdP, ...).
For SAML to coexist with username/password on the same logon page, you would need to start the redirect sequence manually, which is usually not the intention. Therefor an approach could be to have an alternate logon page for non-SSO logon (for example a service on http://yourserver.com/no-sso). This however only work for the logon page, not for apps you directly navigate to. On the other side, once logged in, the user won't see the difference anymore. The only thing to be done in ths case would be to create a different logon configuration for this one app. However rollback is also rather easy, you just have to remove SAML2 from the accepted logon mechanisms of the system and you will get your login prompt back.
You can find a general overview on SAML2 here.
The docs also provide info on how to configure ABAP ICF services and Java Logon Stacks to accept SAML2.
Regards,
Patrick