You should be able to log in as any user provided you know user name and
the associated password. You cannot log in as root in Ubuntu. It is
disabled. You do not have to set up a second account for admin. The default
user can gain root privileges by using sudo or gksu.

You can create an account without access to sudo but would have to do that
specifically. Sudo is password protected and times out after awhile.

Tip: you can gain root privileges without using sudo each time, if you use
sudo su. That way you do not have type sudo before any commands.