Once you have the SSO shared secret you can login with any type of users (Customers, agents, or God too). This means that you must 100% trust the external website.
Would be a nice security feature to be able to create (and revoke) SSO shared secrets and associate any shared secret to an user role, for example:
SSO key: XYZ123
Role: Agent
Means that using the key XYZ123 only users with "Agent" role will be accepted via SSO.
SSO key: ABC5678
Role: User
Means that using the key ABC5678 only users with "User" role will be accepted via SSO.

