I think if you were going to go that route, you'd be best served finding larger tats (say 8-12") and a smaller armatus. Not so small that they'd eat/kill him, but that they could at least keep up with him growth wise. I do think your tank is large enough for an armatus, to answer your original question.
I'm still not convinced that the tats would be able to stay with the Arm, with the ability to hit 2+ feet in home aquariums and the fact that I haven't seen one tat past 12-14 inches I'm still not convinced that this would be a good pairing. Have we seen anyone out there that has coupled these two species together make it work long term?
I'm glad to hear that you guys are confident that my tank was large enough for at least one Armatus. I wasn't too sure considering the care guide says they can possibly get to 30+ inches and my previous inhabitant was pushing close to that mark as well but he was basically a living log and never really moved. I'm sure the Armatus is a completely different beast.
I'm gonna need to pick up some heaters and I have a feeling I'm gonna need a couple of very powerful powerheads. Do you guys have any recommendations for either? I'm gonna need heaters to maintain the 300 gallons at 80 degrees. Been a while since I've needed heaters but do you think 2 500w models would get the job done or should I be looking at something around 3 300w?