Diogenes, you can write a book of logical reasons why to feed cichlids to your P's if you want but lets be honest guys: Most of US (me inclided in the past I admit it) want to feed our P's in such a way just to satisfy our morbid desire to see how P's tear apart living animals.....
I don't agree to feed a big fish to your P's, regardless what you say just because our tanks aint the amazonas river or the orinoco river.
if a Piranha attacks a big oscar in wild the big oscar has plenty of space to escape so the carnage in not commonly completed in most of the cases but in one tank the oscar is persued and it doesnt matter where he goes the P's always find him and tear apart slowly and painfully.
A poor fish facing an enemy armed with razors doesn't seems like a fair match man: Imagine you facing a cougar in a cage just with your bare hands? You wouldnt have an opportunity. What would you think of somebody saying: "well it is the way mother nature works, cougars feed on living preys they are designed to hunt and to kill other animals with their claws and teeth".
So you can say whatever you guys want to justify your morbid desire you're not cheating me with fancy statements dudes, I know what's you thinking at the end, been there. But sometimes one regrets of having done certain things. Moreover now that I am a parent I wouldnt admit my 4 year old boy who loves animals watching my P's destroing a big oscar little by little, definely not a good scene for children.