I have been only keeping Flowerhorns for about 2 yrs now... I do think you can have success if you introduce them at a small size they can be kept with tankmates... Mines (4 - 2 SRDs and 2 Silk Thai) are in a tank with 3 Ornate Bichirs, 1 Redtail catfish, 1 Sun Catfish, 2 Common plecos, 7 Jaguar cichlids, 2 Dovii's, 5 Green Texas cichlids, 6 Convicts and 2 Black Nasty's...
If they are older and bigger< I think it would be a problem as they are used to being alone in tanks...
Okay, I'll bite; what is the size of those fish and also of the tank? Are you saying that combo has worked for two years?