Agree that buying from a 'mixed African cichlids' tank is a bad idea if you want pure species. Most haps and peacocks are still juveniles at 2-3 inches, though that can depend on their growth in your tank and on species size. I bred haps and peacocks for years and disagree that just because some species can start spawning at half their adult size it makes them adults. Thirteen year old teenagers can start making babies, that doesn't make them adults-- and I mean physically adult, not talking about mentally, emotionally, or legally.
Whether or not you can distinguish haps and peacocks from one another, or specific species from one another, depends a lot on species. A lot of them look virtually the same, but fish like Aulonocara stuartgranti "ngara," Otopharynx Lithobates, some of the Protomelas species, and others show differences at relatively small sizes. So, for example, if you're breeding something like Z-rocks lithobates, it doesn't take long before you can recognize them as juveniles.