Can't help with the main question - sorry... but am pretty certain armatus aren't the same as gatf, think they are also known as vampire tetras... (please someone mention if I'm totally wrong)
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Armatus are payara's, specifically Hydrolycus armatus and are from South America. Goliath tiger fish are from Africa and the 2 look completely different. Hopefully someone can go more in depth about this.
For your original question, I've seen the 2 together, but I've also seen an armatus eat an arowana around the same size. It's pretty much hit or miss.
I've seen many Aro and Armatus community tank. Usually Aro is same size or bigger though. Haven't seen a bigger Armatus and smaller Aro in the same tank.
Here are few examples.
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