You can do an all male peacock tank in a 55, but you have to be careful. I'd recommend you stick to the more mild peacocks, which means avoiding the jacobfriberghi group, such as the Eureka red, and man-made species like dragon blood as they tend to be more aggressive. Avoid more than one male of a species and males that look similar. Also, always add more than one fish at a time to an established tank. (once I didn't do this and the fish was killed and I still feel bad about that.) I always prefered males that were mature enough to show color but not fully mature as to avoid/reduce aggression. With my all male experience, I also found that not all fish read their profiles and you will inevitably have to remove a bully and/or a beaten up fish, so having a tank for that ready is useful.