The only limit you have to usernames is that you cannot have digits first, the username must always start with a letter.
In bulk user creation we have allowed the following:
default: First initial, surname, year of entry (students) First initial, surname (staff) Example: astudent07 or ateacher
surname first: surname, first initial Example: studenta07 or teachera
Full Name: full first name, full surname Example: anthonystudent07 or andrewteacher
Full name with surname first: Full surname, full first name Example: studentanthony07 or teacherandrew
All usernames are lower case.