Note that starting from Postgres 10 (which this thread is about), you don't need to adjust wal_level and max_wal_senders (or max_replication_slots, for that matter) anymore. You still have to enable hot_standby=on on the standbys, though.
(and it is in general a good idea to keep the configuration the same as much as possible between primary and standbys).
(and it is in general a good idea to keep the configuration the same as much as possible between primary and standbys).