Master slave replication with different mysql version is on both slave and master
When replicating, the master version must be the same or less than the version on the slave. If this isn't the case, the master may inject binary log messages the slave doesn't understand.

The master version / slave version should stay between on major version. So a 5.6 master is ok to a 5.5 slave but a 5.7 slave is too much, there may be deprecated messages in 5.6 that a 5.7 slave will not understand.

Multiple masters are possible with 5.7 multi source replication..