Do I need to add all campaigns to the Campaign Calendar?

No, you don't.

You can go with the following approaches:

1. Put all campaigns on the calendar.

This way Engage has the best understanding of which campaigns are upcoming and can make sure to reserve timeslots for the campaigns with the highest priority.

2. Put no campaigns on the calendar and just let the rules do the work.

This way Engage cannot prioritise campaigns, and each campaign send will be an unplanned campaign send, where Engage just looks if there are timeslots available for this contact to send the campaign to.

3. Only put the most important campaigns on the calendar.

This way Engage can reserve timeslots for the campaigns with the highest priority. All other campaigns will be considered unplanned and sent if there are timeslots remaining for each contact.