Can I set up SSO just for a subset of my users?
Yes you can!
If you are using Okta you can do this within Okta by granting users access to specific applications. The most common way to do this is to assign users to groups and allow groups to use specific apps (see here for more information).
If you are using Google Workplace you will need to add the groups you wish to sync to Progression before you add your directory.