It's not just about size, it's also about aggression.
Yes, they grow pretty quickly at first and will outgrow this tank in months. Are you planning on upgrading at some point soon, or ever? If not then it simply won't work. If you need to learn for yourself the hard way that's fine, it's just a waiting game - the posts already given should have convinced you. When we buy fish we need to consider how big they grow, not how big they are, and even more so when the fish are territorial killing machines.
2 JD's are not necessarily compatible. A pair of JD's and two JD's are not the same thing. There is a good chance these fish will turn on each other as they mature even if you do upgrade tanks.
As mentioned already there are other cichlids that would actually be suitable for your tank. The personality is something displayed by new world cichlids.
Cichlids have certain needs, and they can't just be thrown together the way we do with community fish. For the sake of the fish do research and ask questions
before you buy the fish, that way you don't have to take them back or watch them destroy each other. Good luck in your decision, I hope it's the right one.
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