Use the forum search and you'll find plenty of people asking similar questions and getting links to good books and such. If you have specific questions this forum is the place to ask them, but "tell me about linux" is so broad a question the answer would fill books.
I don't think you need to learn 9 kinds of linux. Though arranged in somewhat different ways, they're nearly all the same at the core, just with different package managers and high-level management software. My personal reccomendation is
gentoo because it's extremely flexible and has a package manager as easy as apt, but lots of people think it's crazy for compiling everything from scratch.
Plan 9 is not linux, or even UNIX. It's an "experimental" operating system that didn't really take off. It's not dead though, useful bits and pieces of it have ended up in all sorts of things. UTF-8 was originally part of Plan9, for example.
And it's servers, not server's.
I hate iptables too. hate hate hate. I use
firehol to manage my firewalls, it's a language that outputs iptables statements but is far less obtuse and far easier to debug.