yea do the jewel suggestion. try to see if u can get the 5 spot general jewel cichlid becuz they get bigger than regular jewels, and are the most aggresive fish.
My vote for the most aggresive fish would have to be a Buttikoferi, though I've never kept a FH, I have kept(and currently am keeping one) know that they can spank a red devil like nothin'.
My flowerhorn killed 2 pl3co's and the best way to clean your glass is with a cloth as the pl3co's really only do a good job on the tank when they are small. Bristlenose pl3co's are good for that and don't get too big but I guarantee you your fh's will kill any. Mine leaves the bishir alone only cause it lives in a hollow log. She won't let it eat and I have to put the food in the log for it. And my rapheal cat hasn't moved since its been in the tank(lives in another log)