Can a user who is not an admin be added to sudoers to allow them to elevate to root and run administrative tasks only from the command line by using sudo, but not be able to perform administrative tasks in any other capacity?

---------- Post updated at 11:07 AM ---------- Previous update was at 11:00 AM ----------

I think I figured out that the answer is yes. I wonder then, on a Mac, if I give: user ALL ALL will it only allow root tasks to be performed from the command line, or will it allow software installations and other administrative tasks from the GUI?

Yes allowing a user to run only certain commands as root can be done by setting up the sudoers file in an appropriate way.
(Just check that you've setup a correctly formatted sudoers file)