|Keep It Simple, Stupid|
I think that the SiteDocClan have it pretty much under control. Creating an account on PerlMonks, Choosing a username, and Can I change my username? are pretty much all inclusive. I don't want to reinvent the wheel here (or is it already too late?). Certain things could be added to the current FAQs such as "After you create your account and choose your username, why don't you tell us why you chose that username?" then direct the new user to Name Space and/or My username is....
The problem in Forged usernames! seems to have been handled in Choosing a username with the rule "names cannot begin with space or underscore." Other ways to forge usernames might come up, so maybe add a link to that thread somewhere in the current FAQs or information on how to remedy the situation. New users might not know about going to the gods. The same link could be used to report spam accounts such as those mentioned in Curiosity on some new usernames.
On username restrictions, I would include names of core modules to the "reserved" list. I don't know if it would happen, but someone might like a module so much that the person could choose it as a username such as "CGI" or "List::Util."
I do have one wish, that my username would be displayed with a space instead of the underscore, but that is just cosmetic.
Have a cookie and a very nice day!