yeh your tank is big better than what it would usually get, though I just thankfully was able to get rid of the 2 I had rehomed from a 35g and a 55g. I had them in my 300g and though they are suppose to be mostly vegetarian, they began to destroy their tank mates. My beautiful adult male green terror half eaten, A quirky full grown jack dempsy half eaten, a 14 inch oscar and this past weekend I came in to find 7 of my other fish with bites, still alive but definitely attacked.
I do not know why they suddenly turned all norman bates but that would be the day I was gonna let a common 4 dollar fish wipe out my 3 zilli tilapia, my wild caught oscar or my 3 zebra tilapia. So, it was either a kill the pacu's or rehome them thankfully someone stepped up and I was able to rehome them. \
This all being said don't be surprised if the firemouth gets whacked. And don't listen to people that tell you it will stop at 24 inches cause that is not true.
Personally I think a pacu is not a fish that should be in home aquaria as it takes such massive water to keep one happy and healthy. My two seemed to suddenly have gotten a hate on for everyone else in their tank. I watched from where they could not see me at the tank activity and for sure even though multiple feeding stations are used they would rush from place to place where the food was just to scare off all the other fish and not even try to eat the food.
My two pacu were approx 1 1/2 year 23 inch and 1 year 17 inch, and I really am totally convinced that they just took exception to sharing the tank with other fish. I will never ever touch a pacu again unless I have my monster tanks and ponds in place. I have a 430g that is coming, but to me to common a fish to dedicate so much water to when better things can be done with the space.