A few things here.
First, I doubt these fish are imported if that means they came from Brazil. Because they were unable to stop the flow of zebras coming out of Brazil illegally, Brazil resorted to having zebras pu on the Appendix III listings for Cites. Most of the countries in the world are signatories to this treaty which covers the export and import of animals, including fish. Animals on the Appendic III list cannot enter any country which is a Cites member unless it is accompanied by a certificate of origin which states the source of the animal. This pretty much shut down the movement of zebras. However, zebras are now being farmed and there is an endless supply for now, I worked with a partner legally to import a few 100 last March from a well respected Asian breeding operation- very healthy fish.
Second, zebra females can produce eggs every 2 weeks. However the male needs about a month from spawning to the time he release the free swimming babies. This makes the ideal makeup for spawning 2 males for one females. Since only the top of the precking order males and females get to spawn, when there are excess females what they may do is to wait for a male on eggs or wigglers to slip out for a quick bite to eat. When he does, the female will enter the cave and eat the eggs/wigglers. Having lost his spawn, the male becomes willing to spawn again.
Third, I think your fish may be too small to sex. In any case, the only way I know how to do so reliably is to vent them. None of the fish in your pictures have their tube out, so I cannot offer any opinion as to sex. I am actually better at sexing from adult behavior that physical appearance.
