Smoothwall has all the features most people will need in a firewall and is very easy to install, configure, and maintain.
As for Squid, sorry never used it, but from experiance I would say just look for a forum about Squid and go from there. Otherwise I'm sure they've got a man page so just man it.
And I would say give SquirrelMail a try. SendMail is a great package but from what I have seen of it it is massively complex, with very large books having been written on setup and maintance of SendMail.
Irreguardless I believe all of these programs except for smoothwall(which is its own distribution, get the free version) will be available through the standard redhat repo's.
I would like to add to iptables, that it can be managed very nicely with webmin. All you need is to set up some preffered linux distribution (with IPTABLES in it) and set up webmin package. After this you can manage your firewall from your web browser.
1- Linux is only the kernel.
2- The utilities you use are mostly written by GNU.
3- The combination is called GNU/Linux as in Debian GNU/Linux.
4- Firewall is just a software on top of this powerful OS, although many of its features (the firewall features) has roots deep into the kernel.
5- The netfilter is the framework and the library used by the kernel and the user level application, iptables.
6- iptables is just a utility that takes advantage of netfilter framework, hence it is called netfilter/iptables.
7- You can read http://iptables-tutorial.frozentux.net/iptables-tutorial.html in order that you can configure GNU/Linux as a firewall machine.
8- The above item and the link is for mid- to high-level users, however you have the option of reading http://www.higherpass.com/linux/Tutorials/Iptables-Primer/
in order to know iptables more, in an easy way.
9- Also take a look at http://www.iptablesrocks.org/ which also includes examples and sample configurations.