Any cichlid that will stick up for itself is a gamble mate and could end up smashing your aro and one that doesn't is a dead one thats the main problem ive found with aros and cichlids (btf im taking jardini but rtg dont mess around either)
I'd love a get a gar but they're not jardini proof untill about 15"+ even with the armour scales they still get folded in two

I've only ever tried one dat and it got nailed , similar problem with these big one will harass your aro and small one will either stay black and hide or get nailed (they grow to slow compared to aros to) but loads of people manage these combos so its just down to the individual fish and giving it a go mate , like me you'll either end up rehoming fish , trading them or just loosing them to the gamble