Jim I have the 50 gallon tank with 2 Oscars ( that are now huge ) in it and I am getting ready to take it down, they are so nasty you can't keep the tank clean. Then ya have to worry about the feeder gold fish ya buy ( they can have disease ) Both of my Oscars picked up some kind of disease and have been blind for over a year now ( nope no more feeding them live fish
) but seriously they turn into more work than fun.
Now as far as Piranha's go I had them while still living up in NJ ( you can't buy them down here they are illegal ) and they where fun, yes they are messy but the ones I had at least, stayed pretty small ( Piranhas are school fish so ya want a few ) and if ya feed them right I dont remember them being near as messy as the Oscars, Now I remember Sir Evil and myself had went fishing and when we came home we had 17 minnows left and threw them in the tank with the Piranhas, now that was a mess. I think I had 2 or 3 small ones in the tank, so they ate the minnows until full and then just shredded the rest, dead fish floating everywhere
As for the African cichlids, they grow nice, but beware of their little size when you buy them, in a 29 gallon tank 2 or 3 is all you would want to be healthy, although you could get away with a few others ( especially when they are small ) but they grow fast, I had them before the Oscars ( which are basically cichlids as well ) out of the three choices I found these fish to be the easiest to raise ( as far as health ) and they are beautiful fish.
Good luck with them and have fun.