Sorry my answer would be the same for many of the same reasons. The Aro and Gar would make good tankmates for one another. I'll also mention specifically with Armatus my experience is they tend to target tankmates that have more reflective scales such as Aro's. The reflective scales I think trigger a predatory response. Aro's maybe in general slow moving enough to not draw their attention but again the quick movements and feeding habits of the Characidae may cause the Aro to jump.Got it I'll look into rehoming. What about a silver arowana with the fast moving fish you mentioned?
You can try it, I am just trying to tell my own experiences so if nothing else you know what to look out for if you do want to try.