Unfortunately it is impossible to do a peaceful species only tank with the Tanganyikan substrate spawners in that small of a tank, unless they are a compatible pair. They don't pair for life, so if you add other fish, then it is possible that the pair split. One may decide it likes someone else better and it can go south from there. The fish really have to like each other, you can't just have males and females and hope they will spawn.
Small tanks are great once you have a pair that is spawning and successfully raising the fry to certain point. To get the fish to pair off you need a larger community setting of other tangs about the same size and you will see pairs form. Its like they have to be able to defend a territory to get the girl. To increase the pair bond, you may want to let them spawn a few times in this community setting. Then move them to smaller quarters where you can observe them better, and control the fry easier.
Some people say that they have bred N. ocellatus in harems or groups, but I have never experienced that, mine have always paired off one on one. J. ornatus is definitely a one on one pair also, they do not breed in groups. These fish are so territorial that they will easily take over a tank that small and chase everyone else out if they don't like them. There is nothing you can do except remove the fish that are up in the corners, because they will get trashed and even killed.
Here is an article that I wrote on keeping Julies, there is a lot of information that can apply to keeping ocellatus too!http://www.cichlidae.com/article.php?id=422