I've only had 2 flowerhorns the last 3 years. In seperate tanks, mind you and both males.
I have a 10" one that is as mean as all get out. He has killed anything I've tried putting in with him. I've tried quite a few SA/CA's too. He just kills them all.
I have a 6" one also. I got him when he was 2" (I actually got both flowerhorns around that size) and he is the meanest sob that I have ever seen in all my years of this hobby. He hates everything. He "attacks" my dog when she walks by. He bangs his head into the glass over and over like he's trying to break it to get at her.
He killed a piranha a couple months ago, that I've loved so much, when I put him into the piranha's tank for a day to finish the tank he was supposed to be in.
A friend of mine had a flowerhorn that he got at maybe 3-4" and was in the tank with a jag that was 9". After 2 months the flowerhorn was 6" and he didn't like the jag being in the same 90g tank with him and he wanted to let the jag know that. The jag killed him after about 3 days of them fighting, but the thing is the flowerhorn almost seemed like he didn't care tho. It was like he would rather die fighting than sharing a tank with anyone.
Flowerhorns aren't very well liked by most cichlid enthusiasts, and I can understand why very much, but noone knows what these fish are. I've heard that they are hybrids, I've heard that they are genetically enhanced etc. The thing is, noone knows at all.
Alls I know is these guys are terrors! If you get a male, nothing can be kept with them in most cases. They hate everyone, even the person that feeds them.