Good afternoon! I am a SME with SSO for the Cloud. Unfortunately no at this time it is not possible to claim a subdomain and use it for SSO. The purpose of the domain claim is usually so it matches the email address you are using to authenticate.
So your login process looks like this:
1. Login to marketing.adobe.com
2. Select Sign in with an Adobe ID.
3. Enter an email address which would match your SSO that you setup (remember your user account MUST be a Federated ID) if you try to login with an Adobe ID it will not work!
4. Hit <TAB> once it will start the process which we check the FirstName, LastName, Email address and sometimes the Username (note these are the names of the variables you send us in the SAML request.)
Then once it comes through then you are dropped off to which ever landing page you have setup you can change the landing page so it goes to a specific product this setting is located under the Profile settings icon in the upper right corner of the screen and you can change it by going to that icon select Edit Profile and to the right you will see a column you will see a drop down menu called Landing Page. Save this to what you want and then that will be the landing page you come into next time.
Hope this helps! Otherwise let me know if you have further questions would love to chat with you further on your needs!
Sounds good in the meantime to help you and and others on our forms here I will be posting up a topic on SSO for you to read through kind of breaking down the bits of it all and what you can expect so you can get a better idea. So feel free to hit me up on questions on SSO.