Hi,
1. Different companies have different naming conventions. It's common to use something like employee number as a user name. Also it's common to use some combination of first and last name. But be aware of technical limits. See the next point.
2. Minimum is 1 character and maximum is 12 if I remember correctly. You can't have user names longer than 12 characters. Hence you have to take this limit into consideration when you are defining your naming convention.
3. I am not aware of any standard way how to achieve this. I am wondering why you have to do this. The solution does not have to always be technical. Only a limited set of people will have authorization to create a user in the system. I guess these people should be trusted that they will follow the naming convention.
To be honest, if you want to have something robust and future proof you should not be creating users manually. You should feed your users from central repository. Some kind of identity management solution. If that's the case then you probably already have some naming convention defined that you can re-use. Unless you have a problem with 12 character limit for user names. In that case you can work around this using SAML and map users to shorter user names.
Cheers