P. nattereri is not sexually dimorphic (cannot visually tell), nor can you tell by agression at any age. One being more or less agressive is much more likely to be due to that fish's personall behavior tendancies. I have yet to see any credible reports to give such a claim any sort of validity.
Now to the more pressing question: will they pair off. That is totally up to them, the best that you can do is provide the best environment you possibly can. If they get frisky then you will know for sure, but until (if) they do well that's too bad I guess. Like I said before, breeding has occured in a 55 gallon tank on some occasions. Obviously your chances of having males and females that will want to breed is much lower than in a larger tank with more fish, but maybe that should encourage you all the more. IMO getting them to breed in a 55 speaks a lot for the owner and the conditions that are maintained.