How to run elysia cron? Combination with cron.php possible?

Hello,
I don't understand how elysia cron is fired. If I have several cron-jobs configured, do they fire, when users visit the website??? Or do they run, even if there is no traffic on the website? I need to run crons regulary on a not often frequented website. What will happen, if I fire cron.php from a cronjob on my server? Are the elysia-settings respected? Or will this again cause all drupal-cron-jobs to be fired at once?

Background: I have set up rules scheduler to sent appointment-reminders to users per mail. This reminders will be 5 minutes before an appointment starts.
Now I want to run elysia cron to only do the rules-scheduler-cron every 5 minutes. All other cron-jobs will only be fired once a day.
The problem: the drupal installation is on an intranet server. And I want to be sure, that the reminders are sent out, even if no one is working on the drupal-intranet ....