Undo the regex change you made for the cPanel module -- it's invalid.
What action did you perform, and what command is issued via the API that results in that error: "Unable to fetch the cPanel user ..."? You should see it at the top of the input/output module logs, and the input logs should show the username being requested.
When adding new users without specifically setting your own username, it will still generate one 8 characters in length.
If you still experience an issue with usernames 8 characters or less, then it may be the API that needs to be updated. I haven't tested this, but you could try finding and updating (/components/modules/cpanel/apis/cpanel_api.php):
if ( ( $user != null ) && ( strlen( $user ) < 9 ) ) {
to
if ( ( $user != null ) && ( strlen( $user ) < 17 ) ) {
There is no other limitation on username length within Blesta.