It is usually recommended to only use usernames that begin with a lower case letter or an underscore, followed by lower case letters, digits, underscores, or
dashes. They can end with a dollar sign. In regular expression terms: [a-z_][a-z0-9_-]*[$]?
On Debian, the only constraints are that usernames must neither start with a dash ('-') nor plus ('+') nor tilde ('~') nor contain a colon (':'), a comma (','),
or a whitespace (space: ' ', end of line: '\n', tabulation: '\t', etc.). Note that using a slash ('/') may break the default algorithm for the definition of the
user's home directory.
Also calling every one who ever served in 8200 an Israeli spy is ridiculous. Military service is mandatory in Israel. Lots of kids serve in 8200 because they get assigned there for their affinity for math and computers. Most of them do menial Ops tasks, I interview them occasionally for junior positions.
Here are some Israel startups founded by "spies": ICQ, CheckPoint, Wix ...
[1] https://en.globes.co.il/en/article-sagi-bows-out-of-playtech...