Well I have to say the opposite with a lot of you guys (I'm not saying it aggressively, but with my experience).
I have 3 8-9" Tats, 1 6.5" Ray, 1 12" Spot Gar and 1 6" Silver Aro not to mention 1 5" Common Pleco.
The first fish I threw in my tank was the Gar @ 6" with 2 VATF'S. Absolutly not Problems (little over 1 yr housed). Once the ATF'S left (some fish were overlapped with the ATFs), I added the rest of the fish (been about 1yr to as close as 2 months since adding the fish I just said). I have no problem with my gar!!!!!! He and the aro sometimes chill on the top level of water together and no aggression what so ever!!
Maybe I just got a good gar with a melow personality (I hope) but he is the 2nd fastest growing fish in the tank and probly the melowest fish (aside with my feeder sucking pleco

).
Vamp. I think we have the same type of Ray? I say go for it.
And I also say the more time you leave the Armatus solo, the more territorial he will get. An Armatus is a fish that can only be housed solo (until WooChu prooved everyone wrong with 3 Armatus, P/Bass, Hoplias Wolf, BristleNose Pleco, 1 Tat, Bichirs and Huge Goliath all in one Tank, and all had good size!).
A huge Aro is probly the price of 2-3 large Gars and Gars have thicker layers of skin, so should be able to take more (as far as time to move him or heal).
Vamp, I say it is very well worth the try (just like someone had a Puffer and a Ray housed and it was a success. But that is a WAY to darring for me

).