If you want to breed them, It is best to buy a male and a female only.And they should be big enough to tell male from female (my fry were between 4 to 7months old).Some will grow much faster than others.I personally would not buy a small group, they are far more delicate to the differences of the LFS water parameters and yours ,and when they are younger, the stress from transporting is sometimes enough to kill some before you get them home. You are guaranteed to have better chances of a pair spawning when you selective shop older, more resilient fish .If you buy a group of young convicts, not only will there be stress and bullying between at least two of them, but you could all of a sudden have several breeding pair, and that is more than a hand full. And in that situation, the pairs will constantly battle for territory. Not to mention, the smaller the fish is, the harder it is to observe it for abnormalities and diseases. Try buying a male and female at least 2 inches long.Females will have red scales on their bellies just like my picture .The males will be just striped and rather plain with very little if no coloring, and mature males will have a hump on their head that you can't mistake.They are usually more bulky and more muscular looking.That is the best way to start. And it would be even better if they both where bought from the same tank,If not make sure they are close in size. Introduce both of them at the same time into the tank you have for them.This eliminates territory issues. Give them at least three hiding spots. Making two hiding spots close to each other as they will want to go from spot to spot and vise versa until they feel comfortable.They will usually be very active going in and out allot back and forth.Wait till you see lip locking, and the male brushing the female with his tail fin,this is courting rituals. It will get quite rough too, but thats how all of mine have been. I will post a recent picture of my male so you can have an idea of what they look like.Not all females will be as long as mine is, but make sure it has the red on her belly,and sometimes colors as you can see on sparky(thats my females name).
hope that answered your question :)
Female--red spots on the side of belly and generally colored fins
Male--plain striped sides with elongated top and bottom fins
Male--hump on top of the head