The only reliable ways to sex electric yellows (
labidochromis caeruleus) is to either observe them spawning, or
vent them. Eggspots (dots on the anal fin) aren't reliable as both males and females can have them. Males usually have more, but not always, males can also have a white under throat (one of my nicest looking males did anyways), and subdominate males in many speices will often take on female characteristics to avoid aggression from the dominate male.
Red zebras (
metriaclima estherae), males are typically a light orangy/yellow (mature males can have a blue sheen as well), while females are cherry red, though juveniles will look almost identical. In the wild males are usually blue while females are red, but in the aquarium trade you rarely find the blue males, they are often mistaken as two different species!
With
l.caeruleus m/f ratios typically don't matter, as multiple males will usually get along unless you get a rogue. Also note that
l.caeruleus & m.estherae can and will easily interbreed, so if you are wanting to breed them for sale, you'll need to set up another tank and separate the species.
