creating a new user
 

Can I create a user using (1) manually editing the passwd file, (2) linuxconf, and (3) useradd command?

There is more involved than just editing the passwd file, but using useradd the password, creating a home directory and copying the skel files to the directory will be handled for you. There is nothing wrong with using a red hat gui tool to do the same thing. I don't have Red Hat so I don't know the details there.

Administrators might write a script which uses the useradd command to be able to add several users at once.