Well things to consider, looks like you in PA. So no asian aro, lets assume your going with a silver aro. Lets start with a 10" that someone suggested. A 10" silver aro will outgrow a 125g tank 4-6 months depending on tank dimensions. Silver arowana are also very skittish and spook very easy. A spooked aro could also casue the gar to be spooked. Gar swimming at high speed into the side of the tank from being spooked is not a good thing.
So will getting an arowana lead to your gar breaking it's back? Probably not, but it has happened. You'll have to decide if it is worth the risk. For the arowana you'll be needing to soon upgrade to a larger tank, don't know if that is within your means or not.