If a server doesn't list the token, the client will use a maximum length of 10 characters and a-zA-Z0-9._- which are acceptable on all networks.
It looks like freenode accepts period hyphen underscore in the userid, but not as the 1st character.
are valid nicks, but when mIRC strips the backslash to create the userid, these are rejected as invalid username. Unless it creates backwards compatibility issues, underscore and hyphen should be stripped if they're the 1st character of the userid, which means "\_" should change to "user" just like using nick "\\\\" does. The period isn't accepted by freenode as a valid character in a nick, but setting email to ".firstname.lastname@example.org" causes them to reject ".foo" as username.