It is not just about the amount of fish to reduce the aggression but also the types of fish you have.
You have three pretty aggressive types of fish there.The auratus,maingano and red zebra.Cobalts can also be fairly aggressive.The yellow labs are the easiest going in your tank and to date I have not had other types of cichlid bothering my labs at all.They seem to be accepted by one and all.
Generally I would say you could do fifteen to twenty fish in a 55 gallon tank but you need to watch your more aggressive fish very carefully,especially your Auratus.Auratus can be vicious fish and are definately killers.However each individual fish is different and there are acceptions.
I would try to add more of the same types you already have.As Malawi Freak already stated Females if possible but at their current size that could be nearly impossible.You may have to go by trial and error and hope you choose females.If they turn out to be Males you may have to rehome them
I hope this is helpful to you and enjoy your cichlid tank