Can my nickname be anything?
Almost. Yahoo! gives users much freedom of expression, but we are also sensitive to all users. So your nickname must conform to the Yahoo! guidelines.
We reject nicknames we deem unlawful, harmful, threatening, abusive, harassing, tortious, defamatory, vulgar, obscene, libelous, invasive of another's privacy, hateful, or racially, ethnically, or otherwise objectionable.
More information and the full guidelines are in the Yahoo! Terms of Service. and our Community Guidelines.