Having Foreman high available is possible and using Redis instead of Memcache is the way to go as Redis is already used for other components. But typically I do not need HA here as in most environments availability is high enough without it and it avoids making the environment more complicated. And it gets more and more complicated with every plugin you add as not all are designed with HA in mind.
So I would also recommend scaling via Smart Proxy instead of HA.