There are many here on MFK with successful aro/fh combinations. A few good rule of thumb to follow regarding silver/black aros. Following these will minimize problems.
Don't mix the aros with cichlids until the aro is about 10" long.
The aro should be 2x the length of the cichlid.
Keep the aros in a tank that has a length 3x that of the aro and a width/depth equal to that of the aro.
Aros grow very quickly so plan on having a much larger tank ready to go as there is no 120g suitable for keeping an adult arowana.