[Answer]-PostGIS and PostgreSQL search path

That would hardly make sense, since the PostGIS objects are the same for all clients. If every client installs a separate instance, you waste a lot of disk space and also weaken cache efficiency.

If other objects live in the public schema that are of no interest to some clients, it might be an option to install PostGIS to a dedicated schema like postgis and include that in the search_path instead.

If you really want to separate clients rigorously, you’d have to create separate databases or even a db cluster per client.