Its a gamble, I have x2 5" NTT datnoid and 5" Redtaill Giant Gourami in the same tank with no decorations together with x9 baby Indo Datnoid and they live peacefully, no fights what so ever. BTW Do you feed your big datnoid live food or pellets? You might have a chance if you big datnoid has been weaned to eat pellets because they tend to be less aggressive.