Using postgres with the command line without pgAdmin
The PostgreSQL command line client is called psql.  You can do pretty much everything with it (apart from taking dumps, for example), you'd better reading its documentation.  Try running locate psql in a terminal and you will find its binary.

As far as I know, on OS X (and typically on other systems, too) pgAdmin is bundled with the server package, so you won't be able to install PostgreSQL without it.  Here I am not very well informed, so handle this with care.

Regarding the postgres user, on linux machines it is a must to have it (and again, it looks the same on all other systems, too), you simply cannot run the server with other users.  If you don't use PostgreSQL, the user will sit there quietly, won't eat your food and drink your beer, so you need not to worry about it.