Toad supports proxy logins. It should work.
However, in Toad 11.6, Toad actually makes two sessions to perform the proxy connection. So both SCOTT and PDBDBA need the CREATE SESSION privilege.
Starting with Toad 12.5, it's more of a true proxy connection, with only one session. So if granting CREATE SESSION to both users is not an option for you, then upgrade your Toad.