Armatus need large tanks, they get over 2 feet and can be flighty. It seems a lot die from getting spooked and swimming into the glass in smaller tanks.
These fish can not be cramped like other fish either.
The BGKs can mix if they get their full size. Most top out at 12" and at that size would end up food, but if you can get yours larger they should mix fine as long as you get a good armatus.