Hello Bill,
The only restrictions on username charactesrs are that user name cannot contain any of the following characters:
\ / : * ? ” |
As for username size, it cuts off whatever you put in the username box at 50 characters.
And the username of your reseller account cannot be changed after accounts are created underneath it. Unless we make a new account for you but then you wouldn’t have any accounts or settings that you originally had.
Let us know if you have any further questions.
Source: testing myself