I have had cichlids for years and tried many methods to try and get balance in the aquarium. Due to their nature, you will be fighting a losing battle, at least for the most part and especially in a small tank like yours. But there are a few things you can try.
Maybe the easiest thing for you to do is to rearrange the decor and make MORE hiding spots. This depends on how elaborate your tank set-up is. Clay pots work fantastically.
Another option that many people don't consider is to take out ALL your decor. Often times when you give cichlids no territory to compete over, they won't compete. However, this makes your tank pretty boring.
Sometimes if you remove the more aggressive of the two fish, it can give the newcomer a chance to establish himself. I would often use one of those cheap box nets that hooks onto the side of the aquarium. Put the aggressive fish in their for a week, and feed him less than the other fish. Enough to keep him healthy, but you want to 'knock him down a peg.' After a week, put him back in. If he is still aggressive, then take him out for two weeks. Not only does this separation allow for the new fish to establish territory, it allows him to get larger. In general this technique can work well, but takes time and patience, likes most things in this hobby. I have used this method and actually had the fish that was getting beat up turn a 360 and become the aggressor
Remember that cichlids are a pain due to their aggression. It takes experimenting to get a group of them to 'be civil.'
Hope this helps.