Cutteri are more aggressive than cents and they grow fast. Rapps are probably small but by 9-12months of age well fed males in clean water will be a very solid 4". Do to the size tanks you have I don't think dithers are your answer. Give the female places to hide where the male can't get to her and hopefully she'll use it (they're not always that smart). Once the mood hits they'll get along. The standard cichlid breeding conditioning should help too, change the water bump the temp up a degree or two (depending where you keep it) and feed some live food like black worms. I got mine from dogofwar, he told me to give them some piece and quiet and they will breed.