A bit too risky for my tastes but you never know. I'd put them in a grow out to be safe. Unless you don't mind paying for a very expensive meal that is...
I would not try mixing adult bichirs with juveniles.Your juvenile bichirs will be gone in no time.especially when the adults get hungry.Its big fish eating small fish.Its their nature and you cannot change that instinct.So I suggest you refrain from mixing big and adult bichirs with juvenile bichirs.