I've been trying this for something like 4-5months with 3-4". I've done pretty good with 4-5. Anything less than that is like throwing hatfields and mccoys together. Honestly though only thing I can get to live truly peaceful with a devil is a male convict.
4-5 devils don't exactly get along, but the aggression is spread out enough "usually" for someone not get singled out for slaughter.