Flowerhorns can be beautiful and seem to be becoming more common. I've seen them regularly sold at Petsmart. On the other hand, FH' s can be mean as snakes and grow really large. Let's face it, that's a hybrid containing the mixed DNA of some very tough fish.
Your 56 gal
. Tank might be large enough to house one adult Flowerhorn and a small Pleco.
, A 120 gal
. Tank would be the minimum size to even consider other cichlid tankmates. With hyper aggressive cichlids, tankmates need to be somewhat equal in size and temperament, and there needs to be several of them as to dilute aggrssion.
You might consider buying a 1" juvenile FH along with a few other similar sized rugged cichlid juveniles like Texas cichlid, Green Terror, or even a Red Devil cichlid and let them grow up together. Even then your small tank would need to be hardscaped with a lot of barriers and caves as to lessen aggrssion.
That would just be a start. Within six months a much larger tank would be required. I came across a great YouTube which might help "Your fish keeper friend - How I keep aggressive cichlids". That guy is a breeder of aggressive new world cichlids and hybrids and has a lot of knowledge about Flowerhorns.