Replication works across the whole instance. I'm working on a PBX that uses two PostgreSQL instances: 1 for configuration, 1 for call logs. I can replicate the configuration database everywhere and only keep 1 copy of the call logs database.
Multiple databases in postgres fundamentally share the same underlying infrastructure (i.e., WAL), and so do not offer much in terms of scalability or blast-radius protection compared to putting all tables in the same database.