By all means add more peppered, they will cope with 24 it's just not ideal. I have some myself but keep them in my community tank now as I no longer have a temperate setup. If I did I would move them back. It's just best to have a larger group of one species than a couple of each, it's more natural for them to live in large groups.
I have Bolivian rams, a male and female, but they're not a mated pair, and while they're not vicious to each other, they're not the best of friends either. Whereas when I had cockatoo apistos the male and females all got along as they kept to there own space and just occasionally headbutted each other, as cichlids do. You could keep more apistos and have more chance of getting a pair out of them if you add a male and a few females, which makes more interesting tank viewing!