Had a 8 inch devil of a devil. He killed many a cichlid. Wife fell in love with an oscar so I but a barrier up in the tank. The rd tried hard to kill that oscar through the barrier for months, but eventually the oscar grew and knocked the barrier down (according to our housesitter). We tried to put the barrier up (cheap petsmart thing) but they would push and shove to find away around it. Often we would come home to both fish on the same side (rd'd side). We eventually took the barrier out and the fish lived happily ever after.
So maybe if your unsure of a tankmate, try the barrier technique so the fish can get used to eachother. Inadvertantly worked for me.