SAML authentication with SAML is documented at . And what do you mean with "Form authentication"? Is it the "regular authentication" you have on AEM when no SAML is configured? That should work by default, and even when SAML is enabled, too.
The main key IMO is to make sure the resource you are trying to authenticate is "seen" by the proper authentication handler.
In SAML, you can define the paths that, when a user needs permission to access, will trigger SAML Authenication handler. For all others, it should default to the OOTB Sling Authentication when a CUG or ACL is on the resource.