After I installed DC/OS, it prompts a login dialog with login with Google/Github/Microsoft, is it possible to login with a customized user/pass like admin/admin?
Open Source DC/OS does not allow for basic authentication. You can either use OAuth or "NoAuth". You can disable authentication via these instructions: https://dcos.io/docs/1.9/security/managing-authentication/#authentication-opt-out. However, if you go the "NoAuth" route you would not want to leave your cluster wide-open. You should set up a firewall or other means of access control around your DC/OS cluster.