Are there any requirements for the username to be valid?

Yes, a username is valid if it satisfies these two requirements:

• The username should be at least 4 characters long.
• Contains only ASCII alphanumeric characters, or – (dash) / _ (underscore)

There could be exceptions, for instance, the username needs to be at least 7 characters long when adding an account using admin console.

Please be aware that your account username and primary email address are two separate entities (hence, can be different). You can use any of them to authenticate to your account.