Geeze people, designing and maintaining all aspects of a fish tank is a very personal thing. There is no right or wrong way to do it, so long as the fish have a humane way to live. Rudy, if you wanted to get 10 red belly pirranas go ahead and get small ones, but be ready to sell some if they get too big.
It's true that fishkeeping is a very personal thing, however,
IMO keeping multiple huge fish in a tank that is too small is not a humane way for fish to live. If someone asks for advice about that, I will state my opinion. Part of the fun for me is watching the fish grow out, and I get attached to all my fish so I wouldn't put myself in a position where I knew I would have to get rid of some of them later, and put them through the stress of transporting them to a
LFS once again- that to me is less responsible than taking into account their adult sizes and providing an appropriate environment in which they can can grow and thrive.
JMO
That said, Rudy, if you can talk your parents into a 150 gallon tank then you can absolutely keep jack dempseys to their adult size. If you choose this option I would not keep 1 female and 2 males because I think the males would kill each other fighting over the female, but it would be sweet if you had 1 male and like 3 females. Or, if you could get your pH up with crushed coral or whatever, a frontosa harem has always been one of my dream tanks- 1 male and 3 females would be awesome. From what I understand they are not that aggressive, but they are huge and colorful.
If you still just end up with the 55 gallon, I think you would love African cichlids. A Malawi mbuna tank would be very colorful and beautiful, and yes, pretty aggressive too!
And you can overstock your tank to cut down on serious aggression, so you would end up with a lot of colorful fish.
I've heard some pretty nasty things about the auratus!