Can I schedule cron lower than 5 minutes

The cron intervals lower than 5 minutes are strongly not recommended.

There are too many factors that impact the performance of the site and also the execution times between campaigns.

The Max Items option used by fetch saves you from losing items from the feed every time the campaign is executed.

It would have to be a feed that has too many new items every 5 minutes so that some are lost in each execution of the campaign.

Anyway if you still want to change the cron interval you can do by a WordPress filter by calling a function to change ‘interval’ => apply_filters(‘wpematico_cron_schedule_interval’, ‘300’)

Example (not tested)
add_filter('wpematico_cron_schedule_interval', 'my_cron_interval');
Function my_cron_interval($interval){
return 120; //In seconds