Regarding you question "When creating databases to use 'sa' or N'Domain\Administrator' user credentials or a separate [dbo] sql account?"
My answer is none of the above.
Security best practices dictate that logins should not be shared by different individuals due to lack of traceability and, that when responsibilities change, the only method to no longer allow personnel access is to change the accounts password. From an administrative point of view, the easiest solution is to create an Active Directory group, add all of the DBA's AD accounts as members, and then add the AD group to SQL Server with system administrators role.
SQL Server requires that the service accounts and the "sa" login have the system administrators role but no other accounts should have that role. You should be removing these privileges or the logins such as BUILTIN\Administrator. The cluster service account only requires a login and does not require the system administrators role.
After a DBA creates a database ( or a job created by a DBA does), execute the system procedure to change the owner to "sa" or, if using cross database ownership, to a login that is secure, is never used and the password should be unknown to anyone.
SQL = Scarcely Qualifies as a Language