Did you check it doesn't ?

I did with Solaris Express (Solaris 10 should behave the same) and the OS accepts to create user accounts with quite long login names. Those with more than eight characters (not ten) will trigger a warning. The reason is these accounts names might be truncated or simply confuse some commands and/or other OSes. Other warnings will be triggered if the first character isn't alphabetic, if all characters are uppercase or if non alphanumeric characters are used.

That doesn't prevent the account to be created though. Moreover, Solaris accepts accounts stored in external LDAP directories which haven't such a limitation in login names.