Technically, AES can be installed into an existing database / cluster. Remember that in Pega, if you install rules into a shared database, they are available on all nodes, not just the specific node where you mean to run them. In AES case, that means that the AES agents might run on all the nodes in your system until you explicitly disable them via agent schedule

While AES can share database with an application that's not a great practice as AES will impact application performance - mostly when the application itself is already having performance issues. Consider the spiral scenario - application is struggling so sends a lot of alerts to AES. AES services store the alerts into the same database having issues, making performance worse. AES agents query and analyze the alerts using the same database, making performance worse. With more load on the database, the application generates more alerts, which are sent to AES, where they are stored into the database, .....

If you are having issues getting  and maintaining infrastructure  to run PDC (cost, staff, etc) strongly consider using PDC. The PDC service comparable features to AES in areas of governance, performance assessment and trouble-shooting without infrastructure cost and any software maintenance work. PDC itself is enhanced on a regular basis. PegaCloud systems come pre-configured to use PDC service but PDC is available for use with premise and private cloud deployments too.