You can absolutely use only one of the Static IP addresses that we provide.
However, keep in mind, all our proxies are located behind a load balancer, therefore connections will appear to come from different IPs to outside services.
The simple solution for this (if you want to only use one IP Address) problem is to connect to the proxy directly, as this will bypass load balancer.
To connect directly, please use one of the static IPs from your dashboard instead of DNS name of the proxy.
If you're using FTP, if your FTP provider supports SFTP (secure FTP over SSH) then you can use load balanced connections. If not, you'll need to do the above to allow for FTP access.