No, not necessarily. What everyone is speaking of is from pure experience because what else is there to go by? With fish, just as in humans or any other creature for that matter, there are all different types of temperaments and personalities. You can have a red devil, a jaguar, or even in rare cases a dovii, that is completely docile. Where in other cases it could be the opposite. there is no way for any of us here to say FOR SURE one fish will come out on top of the other.... But as you mentioned above, your red devil is 2 inches bigger and on top of that, is already established in that tank. Your jag is smaller in comparison and NEW to the tank. So my answer to you is that it's hard to tell you a definitive answer one way or the other.. But those fish all in one tank is not a good practice to follow.