You cant be certain what will happen, If you do it, make sure you have emergency alternate housing for one of them if it doesnt go well.
A breeding pair of CAs will have a go at anything, regardless of size, and they will claim almost all of a tank upto 8ft i reckon.
Alternatively they might leave the aro completely alone, especially if the aro likes the upper sections of the tank, and everyones temperments are in favour.
Id be inclined to try regardless, just have a contingency ready for the safety of the fish.