I have kept S. pindu and mariae, but not mongo, I could visually start sexing the pindu at maybe 3-4 inches.
The females retained the torpedo shape, and the males started showing more masculine, steep profiles at that time, and same age males were always larger.
Males were quite tough on females so I kept them in 6 foot tanks to lessen aggression, which could be quite strong during a rut.
You can see the steeper profile of a male, and egasturated features, even in these bad shots, he is about 7"-8".
Females remained less steep.
The one below is a young male, maybe 4"
The mariae, beat each other to death before they had a chance to spawn, or even tell gender.
The pindu would produce 12-15 fry, and I had to get them to spit at 12-15 days, or I'd only end up with 1 or 2 survivors.
To clarify that's 12-15 days after noticing the female was carrying.
As soon as the Pindu were ready to spawn, one of a pair would dig a typical "Tilapine type", shallow depression in the sand about 5-6" in diameter.
It's interesting to me how similar the young Pindu looked, to your mongo, of course they are probably only half chromosome of separation.
These were some of my all time favorite predatory cichlids to keep of all time, I always wanted to try mongo, but they were never available, so congrats, quite a coop to get them.