It would be nice if it was possible to set a default protocol and default credentials for adhoc connections.
Additionally it would be cool to have more than one adhoc-field with different settings.
when you're establishing an Ad-Hoc connection and want to reuse the previously used protocol, you can do so by holding down the Cmd key and pressing Enter.
Regarding the default credentials, please adjust your default settings found in the Navigation Panel under "Application - Default Settings" for your desired connection type:
I hope this helps!