The only issue my GATF and aro had was when the aro absolutely thrashed the GATF in a dispute over food. Other than that, they got along famously.
My experience with FATF is that they're quite a bit spunkier than most other ATF. I don't expect the FATF to bother your aro though as long as the aro is bigger to start.
A concern for you might be that the aro will get much bigger much faster than a FATF. The FATF may end up becoming food; but as long as it is fat and happy, that probably won't happen.
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