Hello,
Customers can purchase generic instances to run other software in the same VPC as their AEM instances. AMS runs the instance, keeps it patched, handles backups, etc, but the customer is responsible for the applications they run on it.
You can run tomcat on a separate instance - AMS will host the instance but AMS won't support Tomcat. You will have to manage it, fix it when it breaks, deploy to it, etc.
Regards,
Vishu