If I display all others, do I need to get permission from these agents or only from the MLS?

The display of other brokers and agents listings is covered by the concept of Broker Reciprocity which is an agreement between brokers that they can each share one another’s listings on their public facing websites to help those listings gain a broader audience. This is the basis for IDX. So, obtaining the necessary approvals from the MLS for an IDX feed will cover your ability to share others’ listings on your site. The only time you would need to get permission from other brokers or agents would be if your brokerage opted out of Broker Reciprocity. To learn more, click this link from NAR.