For users who do not have Federated IDs populated, will they be able
to log in? Would they use Salesforce usernames/pwds?
Yes, they'd be able to login via login.salesforce.com with Salesforce usernames and passwords.
It is possible to prevent login via the standard login screen with a bit of a hack - you can ask support to turn on delegated authentication, then mark user profiles as using delegated auth. Delegated auth basically passes the details you supply to Salesforce to an authentication provider that you implement - this validates the credentials you supply and tells Salesforce whether you ar authenticated or not.
You can implement the service to always return a negative, or simply not implement a service at all (just make sure you point Salesforce to an address that is yours to control).
Federated SSO will continue to work, so your users will still be able to login.
There may be another option, because when you set up new users, there's the checkbox to send the email with their username and password - so if you didn't check that box, they'd never receive a password. I'm not sure about what you'd do with existing users though.