We are currently looking into the option of implementing SSO for a community. Is using both SSO and normal login on the same community website supported, or do you have to choose one or the other?
From an SSO perspective no, since it is meant to be a single login process. An option is if the second auth mechanism is Oauth compliant you could create an IOauthClient, like we do for facebook then the user could choose to use the oauthclient while community would remain the primary. https://community.telligent.com/community/11/w/api-documentation/64390/ioauthclient-plugin-type