Replacing Keyhole Cichlids

Reading up on keyhole cichlids, it seems that they are one of those species that are quite difficult to sex when young, and one is advised to buy several youngsters and let them pair up.
In other words, it will be impossible for you to choose one of the opposite sex from the one you have now in a shop tank, and even if you do get it right, they may not like each other.

If you have room, you could try getting another 4 or 5 youngsters and see what happens, but be prepared to take action if your current keyhole objects, and make sure you can rehome the rest if a pair form.