you might put them together, but it all comes down to size, if your aro is too small the Dempsey might kill it , if the aro is too big ,your Dempsey will disappear for sure.
I have a 24 inch silver arowana with two 10 inch flowerhorns , two Jack Dempsey 7 inch each , one firemouth ,one convict , one mean 8 inches Texas,
I never had problems but the small ones have lots of places to hide from the big ones specially the arowana and the flowerhorn