I would not add any bichirs smaller than 18" to an adult size armatus. Since it's only 6" right now it should be fine but armatus grows so much faster than any bichirs. I got 2x 15-18" armatus and they already eaten some 10" fat palmas polli and beauttikoferri. As well as a 12" silver arowana. Best to keep armatus with other more robust species.