Over stocking is key with cichlids, so yes, add more than you normally would stock in a tank (ensuring your tank is fully cycled) and this reduces aggression through over population
Also create LOTS of rock scape in the tank, every little nook and cranny is beneficial to these fish, every hiding place is a safe haven, the more the better.
Take into consideration over stocking means you need to over filtrate, i recommend with cichlids your turning your tank over minimal of 6x per hour (My 450L is turned over 10x/hr with a fx6 and ex1200, + power head)
I also recommend 1.5-2x tank maintenance to keep levels clean, and test daily or every other day.
I also recommend instead of the black gravel i see in the picture you use marine grade coral gravel as cichlids like their ph around 8, marine gravel will set it at 8 and keep it there for you,
I also (I know... another one) recommend the use of Seachems pH and Ammonia alerts, can be found online very easy and will warn you of any rise or fall in pH and ammonia.
As far as sexing is concerned its hard to tell in most cases while cichlids are still young, but one giant give away in most species is their "egg spots" These are the little coloured dots on their anal fin near the tail.
Now males will ALWAYS have these, and some can have more than one (a few of mine have 4+ egg spots) making me 100% sure they are male.
However this is not always accurate as it is known some females can possess 1 egg spot.
Other indications are males are generally brighter and more coloured than females (This is because in the wild males compete for the female by showing their bright colours to attract her, she will pick the most beautiful male she can find to produce her offspring with)
Size can be another factor, and also colouring, some fish of the same species have different coloured males and females, as in the male is blue with stripes and the female is a bland yellow.
All in all, the best thing you could do right now is find some more tank mates for this pair, as it seems you may have 2 males, one will claim dominance and attack the other fish till death.
More tank mates and more rocks is the way to keep cichlids, just make sure you keep up on your water changes and filtration.
For a tank your size also i would recommend the Demasoni, they only grow to 5" max, but like you know already you will need a bigger tank at some point
Hope this didnt make you fall asleep
Paradiddle
