I use flexible daily, and I just disabled a zone from a schedule. I've also added zones into flex daily schedules.
In addition, I have Fixed schedules set up for all my lawn zones for overseeding waterings in the fall. They are the exact same zones that are in my flexible daily schedule.
What are you seeing that is not letting you do this?
The terminology change was something that happened with the 2.6 release at the end of May (see http://community.rachio.com/t/2-6-app-release-flex-app-rach-io-shutdown/4451). I'm not sure if they have all the support documentation updated with the schedule name changes, but the mobile app and the web app UI's reflect the new names.
Just dawned on me -- have you updated your mobile app to the latest software?