You saw that monster goliath destroy that silver aro right? It's so hard to believe that it's the younglings that are the aggresive ones. Why is it?
I have seen that video. I am convinced what we saw was predation not aggression. That aro was probably 1/2 to 2/3rds the length of the GATF and narrow enough to be swallowed whole. The GATF was also in very rough shape and was probably not cared for or fed properly. To that GATF, the aro would have looked quite appetizing.
One GATF eating a much smaller arowana in my opinion is by no means an indicator that those two types of fish cannot be kept together. I've always advised people to make sure their fish are comparable in size to start as aros grow much faster and this faster growth will eliminate the GATF's ability to try to make a meal of it. The GATF in the video was likely 4-6 years old, the aro was likely only a couple months to a year old. That's bad juju and I would have recommended against it.
When dealing with these two types of fish (ATF and SA aros), it is my personal experience and the general knowledge on the site that:
1- ATF, once broken off of live, aren't overly aggressive. They tend to be quarrelsome with fish that looks like them, but are likely to either ignore or be scared of anything that's too big for them to eat. They also tend to completely ignore anything that swims on the surface or the bottom. Granted, big ones eventually develop a supreme air of confidence, but it doesn't mean they will become aggressive.
2- Young silver/black aros tend to be quite aggressive. But...
3- ...Those same Osteoglossids will (normally) mellow out as they become larger. They (normally) only maintain their interspecific aggressive tendencies when kept in congeneric groups.
Granted, every fish is an individual. You can always encounter a fish that behaves nothing like what its supposed to, so of course these are generalizations. I do still feel however that as long as they're broken off of live and as long as the aro is the same size or larger than the ATF to start, this combo is one that will work nearly all of the time.