When the Tablo first came out, you could watch a single channel without a hard drive. But sometime recently it has changed where even a single channel will only last a few minutes.
You should be just fine when you get the hard drive.
Assigning a port just means reserving a DHCP address for the Tablo so that it’s IP address never changes. You don’t have to do it, but it helps if you use the Tablo Connect feature to access your Tablo remotely. If you need help, just list your router make and model here, and someone can help.