you can get the SERVICENAMEs by just pasting http://AMBARI_SERVER_HOST:8080/api/v1/clusters/<CLUSTER_NAME>/services, please make sure that login to ambari UI once before you hit the above URL (to have the login cookie)
In along with that, to wipe out the entire configuration you can reset the ambari which takes to you go to the state that has just ambari.
syntax for that "ambari-server reset" -- highly not recommended until you know you are going to wipe out the entire cluster configuration.
To your second question :
just the agent is enough to install, and you can install the agent from the ambari-server UI ->Hosts -> Actions ->Add New Hosts