I have 3 males in a male hap/peacock 220 gallon around 8-9". They are in there with other large haps, buccos, a frontosa. They are constantly showing who is the dominant one chasing one another but generally don't hurt each other. 2 are showing full color. The most dominant will chase the other haps and peacocks on occasion but not to hurt them. They are beautiful fish. I had one a couple of years ago that was about 12".
I raised a lot of different haps and the champs are one of the faster growing haps. When I got them they were around 2". Within a year they were close to the size they are now. I don't think they would get 18" in a home aquarium unless it was a 400 gallon or higher tank. Like I said previously, I have had a couple that only reached a little over a foot.
I have 3 smaller Bucco Rhoadesii around 3.5" but they aren't a problem, 2 adult male white lips that fight every once in a while, but pretty much the tank is pretty calm. You will have a male fish on occasion that wants to breed and trys to be boisterous but they're cichlids.
Here is a shot of the tank and a pic of the dominant champ and my wild caught b.rhoadesii. Some of the haps are hiding in the rock so the tank looks a little bare. Some of the haps are juvenile so I'll let them grow to adult size to fill up the tank. I probably won't add much more.