You can specify the version of civi you want if it can't figure it out, so e.g.
composer require roundearth/civicrm-composer-plugin civicrm/civicrm-drupal-8 civicrm/civicrm-core:5.13.4
You might also have to add and specify the psr/log version on the same line to match, based on the error message above.
Jon has also answered the original question of how to install CiviCRM in its own database.
Specifically, if you add the connection information in your settings file (before installing CiviCRM), it uses that, i.e. in settings.php add in:
$databases['civicrm']['default'] = [
'database' => 'civicrm',
'username' => 'mysqluser',
'password' => 'a-mysql-...