MYSQL database failover
Mixing MyISAM and InnoDB in ordinary Replication is OK.
MyISAM can screw up in any form of replication.
MyISAM is not allowed in the latest HA techniques -- Galera and Group Replication.
Master-Master, but writing to only one at a time, will let you continue to have MyISAM, and give you quick (perhaps one-minute) failover.
MHA is another approach using standard Replication.


In the long run, you really need to choose between HA and MyISAM.