I would stick with amphilophus, close enough in size and stature to the flowerhorn to coexist in a tank that big. I would't keep anything like the aro, big cats or clownknife as they are more predators than anything and don't have the aggression that the suggested cichlids have, and would probably be stressed. i personally would recommend keeping pretty much a amph community, maybe something like texas and the other medium sized cichlids. It's obviously quite a large tank but when it comes to dovii and umbees, in order to keep them in a community long term, a tank about twice that size would be needed.