Would he be good with oscars and a 3 pacu piranhas?
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Three pacu are going to need over 500 gallons.
You'd probably need more than eight hundred gallons for all those fish... they grow quite fast... oscars would do well for as long as they can hold their own in the tank, but will eventually become dietary supplements for the redtail.
You're going to have to think about rehoming some of those fish...