Removing the zebra will only create a hierarchy vacancy probably to be filled by the auratus which will likely only resume where the red zebra left off (i.e., terrorizing the tank).
Despite the presence of non-cichlids in the tank your zebra is behaving the way it is because your 55gal tank is understocked in terms of the total number of mbuna in the tank(5). When kept in low numbers aggression is enhanced. The zebra (or any dominant mbuna) will instinctively treat mbuna as primary targets of aggression over non-cichlids (which are often viewed as non threats).
If possible, remove the non-cichlids and replace them with 3+ female red zebras plus a similar number of other types of mbuna. Increasing the number of mbuna (i.e., potential red zebra targets) from 5 to about a dozen (including the introduction of red zebra females), should significantly decrease the level of aggression in the tank.
Best case scenario: the male red zebra dedicates its efforts toward attempting to spawn with all female red zebras in the tank, thus leaving the rest alone.
Worst case scenario: the male red zebra behavior remains the same but with more mbuna in the tank, aggression is spread out (11 fish being bullied by 1 have it a lot easier than 4 bullied by 1).