Your 72 x 18 x 29 is to small to even keep an silver aro in. A 20" aro will be over 24" within a couple months, and at that size the minumum size tank it should be kept in would be 180g 72 x 24 x 24. Under the right conditions I do beleive the aro and pirahna could be kept together, but I think this would have to be in a tank that is much larger then most any home aquarium.
Everyone predicts dead aro, even without knowing the size of the pirahna
Everyone predicts dead aro, even without knowing the size of the pirahna

you know very little