User name doesn't have to begin with a letter. It must not begin with a dot or hyphen. Not only letters from a-z are allowed (i.e. ä, é). http://docs.software-univention.de/manual-4.1.html#users::management_6
In UCS 5.2 this section would need an update: https://docs.software-univention.de/manual/5.2/en/user-management/index.html#users-recommendation-usernames