You can’t use it together with an existing hub. You would need to individually migrate any devices from the old hub that you wanted to use with the new one.
https://www.smartthings.com/home-security
Search the forum for discussions of reliability and security systems. All of the other SmartThings model lines are cloud-based, and just not really suitable for a primary security system. Just as one example, there is literally no way to arm/disarm the other model lines without having a cloud connection. You also can’t get any notifications, even to your old phone, unless the cloud is available. That’s why when you now follow any of the links on the SmartThings site to “security” they will take you to the ADT model. It has fewer false alarms and the security functions are also able to work Even if the Internet and power are out, which is not true of the other models.
See the following current discussion thread where you can talk to people who are actually using it: