Once your firewall is set up, all unauthorized traffic coming into your server will be dropped. If you decide to add a service to your server, you'll need to add another firewall rule to allow the incoming traffic to reach the service. Otherwise, external users will not be able to access the new service. In fact, they will not even be able to see that the service is running and their connections will simply time out.
Make sure that your firewall is set up. Refer to the Setting up a Linux firewall recipe for more information. Make sure you know which port numbers and protocols are used by the service to which you want to allow access. Common port numbers such as 80 and 443 for a web server and 20 and 21 for FTP are listed in the file /etc/services. Usermin uses the port 20000 by default.