CentOS

I've installed CentOS on a virtual machine to see if I'm smart enough to use it. I'll want to use it as a server for email, web etc and as a file server for a small office with a handful of users.

The first thing I noticed is that firewalld seems to be running by default with all ports closed.

I'm behind a router with all its ports closed other than those I've opened to port forward to my CentOS server (currently just ports 80 and 443).
So I could either spend a few days learning how to set up firewalld or just leave it off and let my router take the strain.

That would be okay - wouldn't it?

You should always have a firewall even if you have a hardware one in place. For example, what happens if one of your other devices/machines is infected by a virus or trojan? Now they have access to everything behind the hardware firewall...