You can change the login url via configuration: /SecurityConfig/Permissions["Backend"]/LoginUrl and /SecurityConfig/Permissions["Backend"]/AjaxLoginUrl
Your plan is feasible - when you get enough information to judge that a user can login, create a Sitefinity user and log him/her in. To "login" a user, you will need something like
// using default provider
UserLoggingReason result = SecurityManager.AuthenticateUser(providerName,
authenticated = result == UserLoggingReason.Success;
To "logout" a user, you will need
"provider name or null for default"
While not directly applicable to Sitefinity, you might find this blog about OpenID integration with ASP.NET
As far as max users go, we are limited by OpenAccess. In certain cases, it builds "Contains" statements using recursion (e.g 'is user part of that role'), which is limited by stack size (usually 1MB). A few thousand should work, but we haven't determined an exact limit. I have personally tested with 2 thousand users and the system worked without any slow-down or a problem when developing part of the UI for managing users/roles, but things might have changed since. If you encounter any problems due to max limit reached, please tell us so that we might think of a work-around.
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