Hi durgamallesh,
for SSO user to access the REST API, you just need to do as you normal SSO user login via Cookie, it is something like
LithiumSSOClient ssoClient = LithiumSSOClient.getInstance(SSO_KEY.getRaw(), SSO_CLIENT_ID, SSO_CLIENT_DOMAIN, SSO_SERVER_ID);
ssoAuthTokenString = ssoClient.getLithiumCookieValue(ssoid, screenname, email, "", "", "", CALLER_PUBLIC_IP);
the you do HTTP GET for
COMMUNITY_URL + "/restapi/vc/authentication/sessions/login?sso.authentication_token=" + ssoAuthTokenString;
to retrieve the restSessionKey.
for your last question: yes, you are right: with SSO enabled, you can still access REST API with normal Lithium Authentication. However, once SSO is enabled, you are not able to register a new Lithium authentication account. for stage, you may temporarily disable SSO and ceate a new account. For production, you need to log a ticket with our Support team.