Is the intent of this site to stick to a core Unix functionality and not go into topics like firewalls? It seems like there is some cross-over with serverfault.com when you start getting into some topics.
This site technically has 100% crossover with SU (it's a subset of the questions allowed there), so I wouldn't worry about potential SF crossover.
What exactly do you mean by "topics like firewalls"? A question like this I would definitely consider on-topic here:
How do I write an iptables rule to do X
Um... depends obviously it should be related to a firewall on a unix/linux system (PF and iptables comes to mind. no cisco questions) I'd think it should be more syntax than anything. Though personally I think the Network guys deserve there own Q&A site like this area51 proposal. Because networking isn't really Servers. IMO SF and SU are TOO general.