You can definitely use a separate topic for each 1:1 conversation, for example with the naming scheme I described here: Best way to manage Chat channels in Firebase. But there are some things to consider which, as Doug already pointed out in his comment, leads most developers to not solely use FCM for their chat apps.
For example: FCM topics are not secured. This means that anyone who finds out the topic ID can subscribe to it, and thus overhear the 1:1 conversation. And while you can generate topics that are hard to guess, you should not rely on not knowing the topic ID as a security mechanism.
Another reason to consider alternatives is that FCM messages are transient: once they are delivered there is no longer any trace of them. With your current Firestore implementation you can query the database to get all messages to show, while with a pure FCM implementation you will have to build your own database (if that is required for your app).
For these reasons most chat apps I know of use a combination of FCM (for push notifications) and an online database (for persistence) as their backend services.