Thoughts.....
The Armatus are going to need more room then the Dorado. Dorado seem to be very good with boundries, the Armatus have been known to spook, dart, and smash into things.
I personally will never trust Dats and Armatus together again. I had IT and AT with an Armatus that were all fine for like 10 months. Came home to an Armatus swimming all messed up and evidence of a fight Amongst the IT and Armatus. Both were dead within a few days. Wouldn't trust a Jar in there either. The 20+ inch one I had killed many, many fish. I have heard of a case where someone got a Jar and Dorado of similar size to cohab temporarely but the end result was constant fighting resulting in seperation.
I agree 110% with trying. The "what ifs will make you crazy". You never know till you try. The hard part is deciding when enough is enough. I have a tank now with 5 Dorado, 4 Armatus, and several other Characins that's been fine for a few weeks but after several nipped fins I'm gonna start removing the Dorado a few at a time till the shoal weakens enough that they loose thier newfound attitude.
You should post up some pics..... Sounds like a great tank!!!