Newbie proxy question

[Remington L.]

I am interested in installing an FTP proxy server on my default gateway. I
understand caching and all that, my question goes along the lines of
security. Lets say I leave ports 21, 20 open on the server(default gateway)
and I have another machine which is the actually FTP server. I read
somewhere about proxy packet inspection. It is my understanding that without
the proxy anyone can portscan me and find the open port. With proxy
inspection it will only allow in valid FTP commands through, making it
harder to find the open ports. Does this hold true? And I would like to here
any ideas you guys may have on a good proxy, right now im looking at jftpgw

Thanks for your time
-Remi