You'd struggle, to be frank. Dentex are as voracious as piranha while Armatus are aggressive like pit bulls (if you've got a bad one). Between the two species, you'd struggle to keep harmony and in your situation, I'd look to the dentex rather than the Armatus as a 210 won't be big enough for the life of an Armatus. I certainly wouldn't recommend trying them both together. However, I'm not going to say it's impossible... just unadvisable. The Melanopterus is almost irrelevant- unless you have a homicidal Armatus or Dentex, the Mela would probably be left alone entirely or at most, a long way down the pecking order; it certainly shouldn't be in any immediate danger, unless it's small enough to be eaten.
HTH.
This kind of combination is best described as experimental. It's like those in the community who co-hab wolves with others; as succesful as one person's tank maybe, the exact same combination might not work for someone else simply because the individual fish doesn't want to co-hab. I'm working on an Armatus-Curupira-Brycon-Cichla-Hoplerythrinus-Gephagus co had at the moment. Can anyone here tell me whether or not it will work? Probably not but I'm still going to give it a go. And, so far, Curupira-Brycon-Cichla-Hoperlythrinus-Geohpagus seems to be working. Why wouldn't armatus work as well?
If you can, give it a go (with a bigger tank) and let us know how it goes; I'm sure many of us would be interested to know the outcome.