If your red is bigger than the armatus and the armatus is well fed, then you can try. The arowana has more bulk than the armatus... but armatus have one of the sickest mouths out there.
Never had a problem keeping my Arm's with Silvers so I think you should be fine. Arm's don't seem to care about tankmates unless they're small enough to be considered food. At least that's been my experience.
I am more worry about the armatus than the arowana. Armatus are not as tough as their image wants us to believe. One snap from the aro can send the armatus into a death and dying mode..